Situation: I got issued a promask before a field event and it got mixed up with another soldier's promask but my leadership says I "lost it?

Can you get a counseling for losing control of the equipment you were given/issued? Certainly. Just as I would give the other Soldier a counseling for losing control/accountability of their issued mask as well. As long as both masks can be physically accounted for, then really no harm/no foul for the argument of "lost equipment." What the issue at hand is that you both lost personal control/accountability of your assigned equipment. That is what the counseling should cover, with the addition to the counseling that luckily the equipment was accounted for after turn in.

Certainly can, and since there was no lost property it is likely intended to ensure you take better care to ensure control of property in the future.
It would appear to be appropriate for the other Soldier to receive the same counseling, but that's not a call you should worry yourself with.
My point of view is it may also be appropriate for the NCO (TM LDR/SL) to be verbally counseled for failing to check a serialized item while in, and before return from, the field. However, that's just the old school in me, and that wasn't your question, or something you should concerned. Yourself with.
Use it to be a better Soldier.

Yes they can because you failed to maintain control of Your equipment. No different than if you picked up someone else's rifle and turned it in instead of yours.