I apologise if this has already been addressed in this thread and I missed it, but True Strike should work with Witch Bolt. The concentration required for this combo is consecutive, not concurrent: you cast True Strike, concentrate on it and then cast Witch Bolt. You then need to concentrate on Witch Bolt but by that time the True Strike has done its job and wouldn't help anyway. The concentration required for Witch Bolt is only to do damage after round 1. If your DM rules otherwise, you probably want to find another game.
It depends on how one interprets the sentence "On your next turn, you gain advantage on your first attack roll against the target,
provided that this spell hasn't ended" combined with the rule "You lose concentration on a spell if
you cast another spell that requires concentration".
Anything that can end concentration (damage, going to zero hit points, or casting another concentration spell) would appear to instantly turn off the bonus. True Strike appears to have explicit verbiage in it to prevent getting advantage on another concentration spell. It seems that it needs to be concurrent (as per your words) according to the True Strike spell.
Yes, one can try to say that the concentration of the second spell does not occur until after the attack roll of the second spell, but nothing in the rules seems to support this type of interpretation. True Strike also cannot be used for a reaction spell outside of the caster's turn either (because it only happens "on the next turn").
I don't know if this has been mentioned in this thread, but I worked out a way of getting advantage every round. It has its drawbacks, but it does work. Cast Find Familiar and create an owl. Purposefully fail your initiative check as well as your familiar's. Most DMs will allow you and your familiar to go last. Place yourself immediately ahead of your owl familiar in the initiative order, so you're on zero and your owl on -1. On your go, ready an attack such as Fire Bolt on an opponent for when you have Help against it. Send in your owl to provide Help. That will trigger your attack with advantage. Have your familiar fly a safe distance away. Moving away won't provoke an attack of opportunity from your opponent as owl's have Flyby. Rinse and repeat.
That's actually pretty clever. I wouldn't allow it in my game because I think that the game designers purposely went out of their way to prevent players from moving their initiative in order to "game the system" like this (there have been threads here about Rogues lowering their initiative so that they can act after the fighter in the first round). And actually, I cannot find a rule that one can purposely miss their check or drop it to one. I think that the game supplies a mechanic for lowering one's initiative. It's called readying an action.
So, I would allow the wizard to roll normally and the familiar to roll normally. If on round one, the wizard beats the familiar's initiative, then he can ready before the familiar on every single round. If the familiar's init beats the wizard's, the wizard's round one ready action occurs during the familiar's turn on round two. The wizard PC just does not get to control who gets to do an action between himself and his familiar. He cannot force a situation where no PC or NPC can possibly act between himself and his familiar without them readying an action. That's gaming the mechanics.
