You're in the wrong area but I'll answer your questions anyway. Maybe you'll get moved. Maybe you'll read the rules of the forum.
Yes, all models with Krak grenades can use them in close combat. They only get one attack each though. Against walkers they would be hitting against front armor, not rear. Yes it makes vehicles and MCs a little easier to beat.
Against single model units the focused witchfire hits that model no matter what you roll on your psychic test (Assuming you don't roll over your Ld). You cannot use powers against vehicles that require Ld checks to be made by the vehicle. If the power does not require a Ld check, but simply gives you control then you can use it. If you target a transport with Puppet Master, you are not targeting the unit inside. Therefore, you would not gain control of the unit inside. You also cannot target the unit inside, instead of the transport, because they are considered out of line of sight while in their transport.