The area where you find the key is a sanctuary
What key? O_o
a place of power to Silvanus. Druids would go there to connect with him more
You mean more than in middle of their Grove, where they have statue litteraly radiating his divine power? O_o
The rest of the secret tunnels was a nexus for druids, in the beginning. They could maneuver from the heart of the grove to the forest outside to the caverns and the waterfall entrance relatively easily and quickly through those tunnels. And so, it made the lives of the druids back then easier.
Yeah ... so they could if they simply walk out the main gate and take few dozen steppes. o_O
The doors were then imbued with magic runes. Only those who knew the Animal Friendship spell could pass. Thus, you would have to be a friend of nature to be able to enter the secret passages at all. The stones would not roll away unless someone knew the Animal Friendship spell. This would limit the tunnels to only druids and rangers and such.
And Bards, Sorcerers, Wizards, Warlocks ... and pretty much litteraly anyone who have the scroll that can be buyed for few dozen gold and the ability to read it.

(Moonbeam makes way more sense for druids than fire bolts.)
No it does not ...
This isnt World of Wacraft, DnD druids are pretty familar with fire.
https://roll20.net/compendium/dnd5e/Produce%20Flame?fromList=Produce%20Flame&Name=&Classes%5B%5D=druid&Damage%20Type%5B%5D=fire#content
https://roll20.net/compendium/dnd5e/Flaming%20Sphere#contenthttps://roll20.net/compendium/dnd5e/Spells:Fire%20Storm/#h-Fire%20Storm
https://roll20.net/compendium/dnd5e/Spells:Wall%20of%20Fire/#h-Wall%20of%20Fire
https://roll20.net/compendium/dnd5e/Heat%20Metal#contentShapechangers, like shadow druids, would be even weaker to these, for they would get disadvantage on resisting the radiant damage. They would also be instantly changed back to original form the first time they were hit.
Shadow druids is just name ... they are not made by shadows, so there is no reason why they would be any weaker to radiant damage. :-/
In order to allow allies through, the druids then made it so regular animals were fine
So any enemy can simply watch a squirel that just revealed secret passage ... brilliant. -_-
and so that anyone wearing a Circlet of Nature could pass. These circlets were special to the Emerald Grove. So, any druid of the grove could walk freely in the tunnels without the statues harming them.
And since you are talking here about war with dark justicars, those circlets would somehow destroy themselves automaticly when their wearer die, or is defeated in battle i gues? O_o
Bcs otherwise you just locked your door, and hanged keys on the nail that is outside.

But, then the druid leaders reasoned that if a large enough army found the secret entrance, the statues wouldn't be enough. Therefore, in the dark passages they purposely spilled oil along the main paths, so that if enemies in large numbers came down there, they could be set on fire more easily.
And caused natural catastrophy when the oil soaks to the ground and kills litteraly every plant that is there ... just like oil usualy does. o_O
Those leaders were worse druids i have ever heared about.
1. Statues shoot moonbeams instead of fire bolts.'
2. Statues can't be destroyed by arrows or even hacked up with swords.
3. You can't even get into the secret tunnels unless you have a Circlet of Nature or are able to cast Animal Friendship, making the area more of a reward for nature lovers or thieves/murderers who get their hands on one of the circlets.
4. Statues don't harm characters and any allies/enemies within 200 feet of the one wearing the Circlet of Nature.
5. Animals and familiars can move past the statues without getting harmed at all. Ever. This makes familiars and animal companions even more valuable as scouts and such.
6. Statues can be disarmed via the Dispell Magic spell. That's what the spell is for, after all, to dispel traps like this. It still works with the defenses because only IF you have a spellcaster with Dispel Magic and only IF you have a spellcaster with you with Animal Friendship, and only IF you weren't caught off guard by the statues, would you be able to use Dispel Magic on a statue. And then, even then, you might only be able to use Dispel Magic on like one or two of them; certainly not on all.
7. You can't just sneak past the statues. That's dumb. What good are these guardian statues if you can sneak past them? What invading enemy is going to NOT be sneaking when they are entering their enemy's domain?
8. Statues don't set off oil spills and blow you to kingdom come.
9. The key to the chest isn't on some dead guy across that jump down in that alcove. The key is either on Findel or in some hidden nook where the druids hid it.
10. That alcove is a sanctuary to Silvanus which maybe gives some bonuses to Nature, Healing spells, etc. for the remainder of the day. Can be used once a day, thus making the secret tunnels a rewarding area to unlock.
11. The Storage Area in the tunnels is clearly only a temporary storage area, and not meant to be a permanent one, like it seems to be currently. The chest can't even be moved with the staff inside right now. It's like bolted to the floor. So maybe Findel can explain that the "storage area" is only there because the druids are trying to hide some of the best of their things from the tieflings in a place the tieflings can't even go. They're doing this because the tieflings are taking all their stuff; especially the little kids who have formed their own criminal thieving ring.
12. Throw in a bit of backstory with some sort of documents you can find about the secret tunnels describing the things I did above, and we're golden here. The secret tunnels would make more sense, provide players with a challenge and several options to get into them and through them, the traps would use druid magic, they wouldn't be able to be destroyed BY ARROWS, and players would get a good reward if they can figure out how to maneuver through there without dying (meaning they could get the Silvanus sanctuary buff for their party.)
1 > Nonsence.
2 > This i actualy like, its really odd that stone statue can be destroyed by bunch of arrows. O_o
3 > So all those changes and lore Larian would create for this zone, would stay hidden for most people? o_O (Yeah, you guessed it ... nonsence)
4 > This is the worse idea in whole topic ... 200 feet is simply too much ... imagine that some druid was chased by enemy, and seek refugee in this tunel ... now he have to somehow get 60m ahead of his enemies so traps even activate

and if he dont, he just helped enemies to get past all security this grove have.

If there should be some "Save distance aura" it should not be more than 3m radius.
5 > [sarcasm] I actualy like this idea ... you get inside the passage, you spot the statue, you are not sure if that is safe to proceed, so you send your familiar to scout ahead ... seeing he proceeded unharmed you go there too ... and you are reduced to cinders and ash imediatly.

Brilliant!

[/sarcasm]
Seriously tho, no ... this isnt even nonsence, its regular bullshit ... if you want to create secret passage, you dont want animals to spoil its locations, and if you dont want animals to get harmed by your traps there, you simply try to create it so dont allow them to enter.
6 > I like this idea, seriously ... im not quite sure if dispel magic would be strong enough to disarm them permanently, but lets say "until the long rest" sounds quite fine to me ... statues would need to be glowing all the time tho, so players can clearly see if they are or are not deactivated ... and i would certainly allow player to deactivate them all (one by one ofcourse) ... if someone wish to waste all his spellslots for such action, he should be allowed.

7 > Agreed ... i mean i would not see it as such horrible idea, since many (not all tho) security asets we are actualy using in real world are set to ignore everything under area of belt. So personaly i would say that this needs to be unified ... either ignore everything under certain high, or attack everything no matter the high ... but in general why not ... this is good suggestion.
8 > Im affraid in that case those oil splits loose their main purpose ... unless there is allways some guarding druid with prepared firespell.

9 > OH that key.

Yeah, i would not mind if Findel have the key ... but i still believe mine version is better. :P
10 > Again ... you mean more sacred than middle of the grove, that is litteraly looking like some temple, with statue that is also litteraly radiating divine power from Silvanus himself? O_o
I mean ... if we look past this, some well with magicaly infused water, altar or long forget statue would be nice ... but "sacred tunel"? :-/ That sounds odd.
11 > This one i like, but i dont think Findel needs to explain it so complexly ... they are just hiding things here ... nothing more needs to be said.
12 > Funny you mind paper in Blighted Willage that somehow survived for 100years ... and now you want to throw in some documents that will provide backstory for this. Is that some "sacret cave of exceptional paper preservation"?

Nah, sory ... this place works better as something most dont even know about, and some allready forgot.