To fool an evil God

[Entessa]

Players meeting Kazgaroth.https://forgottenrealms.fandom.com/wiki/Kazgaroth

He gives them a chalice - when they kill Bhaalspawn, the chalice will get full of the enemy Bhaalspawn essences.

1) I would like the player to backstab kazgaroth. Would it be possible to deliver the chalice to someone, to make the "essence" poisoned/Weakened in some way so that Kazgaroth doesn't realize instantly when it devours the content of the chalice, but after some time, he realizes something is wrong? Who would be the right God to ask such a favour, considering that the players are evil? How could they get in touch with such God?

I think even a good God would help, though.

1.1) Would a high level druid be able to sabotage the chalice?

2) Malar followers. Any suggestion on which quests they would assign the player? More than one, if possible.

3) Kazgaroth followers: any suggestion on which quests they would assign the player? More than one, if possible.

4) The players will be tasked to drain a druid grove. If they succeed, and rather than delivering the item with the "life/power" of the grove to Kazgaroth they want to consume it themselves, what bonus should the player that consumed the item get?

[Kardwill]

1) I would like the player to backstab kazgaroth.

For this particular point, your players will have their own ideas and plans. If you want them to backstab Kazgaroth, just accept any reasonable plan they come up with as a valid one (and thus, if they ask "can we poison the chalice with the help ox X?", then yes, X is able to poison the chalice, if Y condition is met).

I think it will go better than trying to find a plan yourself and imposing it on the players, because
a - it will be their plan
b - they won't have to play "read the GM's mind" to come up with the GM's solution
c - 4 players pooling their brains can come up with plans a GM wouldn't think of
and more importantly
d - If a GM comes up with a "perfect plan", he's less likely to listen to his players' ideas (because we're usually invested in our own ideas, and want to see them play out during the game. So we tend to instinctively dismiss anything different).

For the rest, I'm not a Forgotten Realms geek, so I'll let other respond to the other questions. ^^