See from what i understand in your case, they are first trying to get you a engineer's visa which might be taking time to convert depending on your company's quota for visas. Secondly unless you do your medicals and clear the finger printing you will not be under their sponsorship, you will NOT be able to go for medicals or finger printing until they get you the engineer's visa. so you see its all inter connected :P

Best option is ask for your passport back and tell them you are quitting..no need to make excuses or anything.