Getting their favor would mean getting a pretty large and powerful group in our corner-pretty handy against the absolute.
Thats quite some asumption there ...
Why would they own us anything? Their plan was working perfectly by itself, we merely speeded things up.
What kind of lunatic would risk his life for someone who managed to do soething that would happen anyway? :-/
They could give their insight/quest rewards/etc and/or send kahga or whoever to tag along in place of Halsin.
Or they can cut our head off ... bcs they are radicals, and we are turning into unnatural abomination.
https://forgottenrealms.fandom.com/wiki/Shadow_DruidsAlternatively, if we do their dirty work for them, the rite is unnecessary, since their goal was to isolate the grove so they could weed out the druids that didn't see ideologically eye-to-eye with.
You seem to forget all those attacks on the Grove everyone is talking about ...
That is another thing ritual should protect them against! Plus any future traveler that would like to settle there aswell.
And even if it would be unnecessary, what reason would they have to not finish it?
They want to do it, they have everything they need, there is nothing stoping them ... unnecessary may not be even near to good enough for them to stop.