A couple of points:

1. if you purchased this ticket through UA/LH then the connection is "legal" even if they changed the min conx times. If you misconnect UA/LH will have to reroute/rebook you on the next flight. Dont let them tell you otherwise. If you bought this elsewhere they should still work with you but then it may depend on how you work it.

2. from A (Schengen) to C is def possible in 55 minutes. I flew LH from AMS and connected to UA at C4 in under 30 minutes the other day but it was around 400 in the afternoon and the lines were short. BTW if you are leaving out of C1 to C8 there is no second screening just the the main one at the entrance to the concourse. so given an on time arrival and a favorable gate you should be OK.