I'm just throwing in some arguments and opinions that might come up during the debate, for leaving and staying...
Leavers say:
- 'Valinor is fantastic'. The ambassadors will describe the beauty of Valinor, the magnificent Two Trees, the light, the gardens of Lorien etc.
- 'Middle-earth is dangerous, we need help'. There will be talk about the Hunter and other potential dangers. Who knows what will happen? We are safe in Valinor.
- 'It is our destiny to live with the Ainur'. Ingwë will speak of Manwë and Varda and all that they can teach the elves. Finwë has met Aulë and speaks passionately of his smithy and the things he does there.
Stayers say:
- 'Middle-earth is fantastic'.
- It is our destiny to live in Middle-earth.
- 'Look, you've been gone for a long while, we actually forgot about the Valar and all that. We have settled down, we like this place'.
- Who will protect the lesser creatures from the shadow if it returns?
- 'We don't need help'. The Hunter is gone, and we can handle basically every other possible danger.
- 'Why can't the Ainur come over here?'
- 'The invitation is a trap'. Some elves will still be afraid of Oromë, and perhaps think that the ambassadors are under a spell or something.
Some might also actually suggest a division, that those who want to leaves and those who want to stay does so.
Among the Avari, we should have those who are actively opposed to leaving and who have some really valid arguments, but also those who are just scared and don't like change. Also, I think we could introduce Eöl here. He could argue that the elves don't need help. He could be an Avari who stomps off during the heat of the debate, enraged and fed up with everyone.
I do not think that the Valar should spy on the debate. The primary reason for this is that I think that the Valar believe that, since the ambassadors are all super excited about the whole thing, everyone else will be as well, so they haven't anticipated a debate. I think it would be great if the debate erupts as kind of a surprise. If anything, the ambassadors are thinking that it will just take a simple description and their own enthusiasm to convince everyone to come along on the Journey. And the Vanyar are all for it, so maybe, initially that seems to be the response. When the ambassadors return, the light of the Trees have changed them, and many want to follow them and make them kings or at least leaders .But then someone begins to question the idea - probably one of the more prominent among the Noldor - and that opens up a door for others to follow with arguments against leaving. I don't think it should be an organised council at first, so if that happens, that should come as a result of a more spontaneous argument when the ambassadors return.
About drama: I don't think we want any kind of physical violence here. That comes later, in Valinor. But these are the events that lead to the first division, and beyond hair colour and favorite pastimes, there should be other things, so we need to add something more powerful, some differences in core values that become clear in this episode. I don't know exactly what those should be, but in any case, they define the four groups who become the Vanyar, the Noldor, the Teleri and the Avari and tie the members of each group together.
One thing that would be dramatic is that the debate could end with the Avari basically saying to the other that they should leave, that that they almost kick those who want to leave out.