Dunno about the first question but to make the altitude constraint given by ATC in your second question, and assuming a Boeing FMC, I'd insert the waypoint "TNT/-20" in the LEGS page (the -20 means 20 miles before) before the waypoint "TNT" (assuming TNT is on your route and in the LEGS page). Next I'd put "/XXX" opposite this new waypoint on the right and voila, the VNAV mode will try to make that constraint.