Now add all the effects from items and coatings that you have at that point in the game.
Now add in all the effects from other items to the Warlock too.
The fact is, Warlock will outperform Rogue for damage. It's not even close.
So to keep this on topic (barely): it doesn't makes mechanical sense to trade Wyll for Shadowheart, Lae'zel or Astarian at the grove. Would you prefer level 3 warlock or a level 3 rogue in a 4 person party? And Astarian is the only viable trade - getting rid of the healer or the tank doesn't make much sense.
It makes perfect sense.
Astarion's only thing at this point is being Dex based so can do locks/traps. But according to you "dexterity is the one stat that rules them all." so why isn't Tav pumping Dex thus obfuscating the need to have Astarion? (Honestly, it's not hard to have a Tav with respectible Dex, Chr, Sta AND their primary stat of choice such as Str/Wis)
Shadowheart can go because healing sucks. Far, FAR better to prevent damage than heal it. Which Warlock can do via actually killing things (Which Cleric cannot do until Spirit Guardians) and Darkness/Hunger of Hadar.
Lae'zel can be traded out because anyone can be a "Tank" since the only real requirement by the grove is a shield and some medium armour (Since the heavy armour at this point still has the same AC as the Githyanki half-plate Lae'zel starts with or the Breastplate +1 you can "Borrow" from Dammon). Later on heavy armour becomes more important... But hey guess what? Shadowheart is a Cleric and Clerics also can wear heavy armour!
Then of course, you're already making decisions about composition by this point since by the time you reach the grove you can have encountered Shadowheart, Lae'zel, Astarion AND Gale.
you also miss out on the synergistic effects of the items that Larian has put in the game.
Which suck.
Like the best one is the one that provides spellcasting stat to the elemental damage of your cantrips. But it's still just worse than Eldritch Blast because it already double dips on spellcasting stat by the time you get that item.
The game is full of items that boost damage from cold, poison and fire damage.
Most of those only affect SPELLS not CANTRIPS though. Honestly there aren't that many items that even boost the damage you deal. There's what, the various items that grant Heat and? That one robe that gives a bit of damage to Poison (Ignoring how crap the Poison cantrip actually is)
Even on tactician there's no time pressure in the the game if you've been looting all the food and, again, not resting often enough can mess up romances and story progression. Even if you are giving yourself time pressure to maintain immersion the restoration fountains, potions of angelic slumber and spellcrux amulets will restore missing spell slots.
It's not even about time pressure. It's that Eldritch Blast actually does more damage than many spells. Like, outside of your big AoE spells and some specific things (Like upcasted Scorching Ray as Fire Dragon Sorc/EvoWiz or upcasted Magic Missile as EvoWiz because their spellmodifier as damage affects each projectile on those attacks) it'll often be simply better to just Eldritch Blast something, irregardless of spellslots.
Guidance is a crap cantrip?
For dealing damage? Yes.
For utility? No, but then it's not even unique to Cleric. Druids can get it. WARLOCKS can get it. There's an item right outside the Grove that gives you it.
Have you tried insect plague?
Yes. It sucked compared to upcast Spirit Guardians.
Is decent. But is a 6th level spell. Meaning you've gone through the ENTIRE game to get your second not useless spell?
I admire your commitment to the discussion but if I had to choose the one trick pony from a line up I would point at the warlock before the cleric
"I cast eldritch blast"
Warlock is a one-trick in terms of action usage. Cleric is one-trick based on what they overall can provide to a party.
Cleric provides a few buffs and a couple of decent spells. They provide little damage, control or utility.
Warlock provides damage, control AND utlity. Even if most of that is simple provided by Eldritch Blast spamming.
In the end, Wyll only loses out because he's Wyll.
Wyll doesn't lose because he's in the Grove. He doesn't lose because he's a Warlock. He loses because he's Wyll.