I'm still not understanding why you're so strongly doubtful on this. Overthecap.com anticipates that there will be nine 3rd round comp picks this year, and Alex Mack is ranked #7 in that bunch.
They do agree that he's on the bubble though. They believe top 5% of salary is ranking 105 -- which is all active roster and reserve lists (i.e. IR and PUP players). If you only included 53 man roster, the threshold would be #85. Mack is anticipated to be ranked #87.
Projecting The 2017 Compensatory Draft Picks | Over the Cap
I know it's uncertain, but in the words of our favorite investigator, I think it's more probable than not that Mack qualified for a 3rd.
That said -- I don't disagree with you that if they work out a trade with Cleveland, they could possibly simplify a few things and combine the Collins 3rd rounder and the Mingo 5th rounder into one package.