Firstly, what a great idea and your ability to follow through is truly remarkable, Flugente.

A few quick ideas which you may have already considered:
Would it make sense to increase the chance to improve in marksmanship / combat level for the spotter if targets are hit?

IIRC snipers (whether it's OCTH or NCTH) don't get accuracy bonuses for repeatedly firing at a target. Would it make sense for a bonus to accumulate if a sniper is firing repeatedly at the same target that has been spotted? (Accounts for the spotter helping the sniper dial in his/her aim)