I have a question about Compel. +20 to the enemy's opposed WP test aside, what happens if the psyker is able to comel the target to take his bolter and shoot himself in the head? Would you bother rolling for BS or damage, or would you do damage differently?

I mean obviously he's not going to miss, so personally I might just wave the BS roll. And I might even change the damage roll since it's a point blank shot to the head. Maybe Have damage go straight to Critical (bypassing wounds), and have damage reduced by armor and toughness as usual. Possibly have the damage roll its maximum amount automatically.

Any other ideas or queries about any other psychic techniques?

No other questions, but handle the situation like every other bolter to head situation.

If the players manage to level a bolter to the head of an enemy, its exactly the same situation. So if you wave the attack roll once, always wave it. Unless the person in question has some near magical agilty....

I'd waive the hit but still do the damage roll. There are rare occasions where a gunshot to the head is not fatal. Especially in the case of compel, on the sub consious level every creature instinctively tries to keep itself alive.

I'd do the roll, using it for minimum damage. +30 Helpless, +30 Point-blank, that's still almost a guaranteed hit, provided the weapon doesn't jam.