....the warning was about Broken Valley not Sentinel Islands as far as I can recall. But one can only assume the inevitable fate of both game Zones once he/she realizes that Divinity II is not an open ended/sand-boxed game and follows a rather arcade pattern......
... and that is the warning I heeded, i went back and finished a couple of things, but it did not occur to me to do the same for Sentinel Island, quite the reverse since I assumed it would be my domain now.
I can see why Broken Valley quests become dead since Damian invades it and it has to be reset, but I can see no good reason why the same should be true of Sentinel Island and certainly not without a warning to that effect since an important component of a key armour set is there. Bad design IMHO.
The game does not seem sure if it is sand box or linear. It plays
very sand box except where it suddenly does't and sometimes the sand box element doesn't seem to work that well.
Example: I hit Keara's FF first (simply 'cos it was marked on the map which I took as some sort of hint) at level 25 i think. I had a week's worth of hard and exiting fighting there, it was the highlight of my game, epic, and i was getting 2300 XP per kill to start so I advanced I think from 25 to 31 in that one extended battle.
However once I'd done that all the other FF, invaded Broken Valley, everywhere became very easy and a bit boring. I was getting only about 80-130 XP per kill which at that level hardly touches the sides.
I'm just doing the Broken Valley Mine now before going to what i imagine is the finale at HoE and finally I've got a bit of oppositision since Keara's.
I am not a fan of auto-leveling monsters to the player level, I use Obsidian's mod for Oblivion for example, but i do think the player should be encouraged to attempt main quest line areas in roughly ascending order of difficulty. This is normally done by having the main quest split up into a lot more stages. Here in DKS there is just one main quest per main area, and this is arguably a design flaw.