1. You never want both charges of an Assault off cd but beyond that the 2nd charge can be held until you move or need to weave an ogcd. Make sure you won't miss out on any charges when going into Bahamut or Phoenix and that you don't waste any Ruin 4s either.

2. All Trance does is reduce cast times so really you're free to use it whenever. Being in Trance helps if you need to move a lot, otherwise you can Deathflare whenever is most opportune: Finishing off enemies, after getting dmg buffs, when you're ready to go into Bahamut (Assaults expended + buffs if possible) before a boss goes untargetable, etc.

3. You have a lot of time to use it so again if you can hold onto it until you get a buff that's good, and be wary about using Ruin 2 or 4 just to weave Fester if you can help it (Ruin 2 being a dps loss and Ruin 4 being better saved for Bahamut and/or buffs unless you have extra you need to spend), better to fit it in naturally than forcing it. Otherwise just make sure you don't have to delay Energy Drain or waste any Aetherflow.