What is the wait time before I can hit a Titan after joining a new alliance?
Have a look here
So if that’s the case and if I understand correctly, what’s the benefit to a merc helping kill a Titan?
Not much/not any as far as loot goes.
If you are VERY careful with the timers and everything aligns right you can merc around and get a full loot titan once per day like you do in an alliance … or go get a full loot titan during a scheduled pass titan in your alliance. (And if you aren’t careful your loot will be awful)
But you can have fun with titan flags you don’t need … or you can go on a world tour of various alliances … or you can just bounce around without a long term commitment to any alliance.
@Dwitehawk we missed your original question.
You can hit immediately upon joining as long as you have a flag and there is a living titan.
You will probably get reduced loot.