No. You have to clear all your subjects to made it to Class 12th. Rest its marks does not matter. 

No,it's not important at all. For placement you just require good skills. Be updated with the latest technologies. It will help you in placement. 

Your communication skills should be good. In placement session, you have to go through the GD(group discussion) round. So focus on your communication skills also.

Your BTech% matters for the placement. It should be more than 60%.

Good luck!