@Curt Bixel,
Eat the cost and move on. Consider it an investment in your own education. My quick read on this is that you need to ratchet up your sense of control.
Alternatively to eating the cost and moving on, and depending on how much you want to burn the bridge with Contractor A, you could ask them if they can issue a full/partial refund based on the pictures and Ditra install requirements, and if they refuse, ask how much do they think a negative review will cost them in lost business - especially since you'll mention Contractor B fixed A's mistakes. You might also mention that you won't be able to recommend Contractor A to your other investor friends, but you can and will recommend Contractor B. Deliver this message with kindness in your voice, and if you can muster an empathetic tone (empathy b/c Contractor A might lose future business), it can help. Ultimately, it doesn't seem like you'll get far with Contractor A, but you never know. You might feel better after.
And yes, mastic over Ditra is a huge no-no (longtime Schluter fan here).
Here are some of the takeaways...not rookie mistakes, but learning moments aka opportunities for improvement:
-Trust but verify = During any remodel effort, check-in frequently - daily or 2x daily depending on the speed of the work. Or make sure a trusted person is checking in that often. You would then ask for written updates from that person after each visit.
-If you're giving the PM that much control, make sure they have incentive to do a good job. It wasn't clear to me what role the PM was playing during the remodel; if they were acting as the GC, then they should have paid the contractors, and then you pay the PM upon satisfactory completion of the work. Under this model, you likely wouldn't have paid the PM after noticing the shoddy work.
-Pay deposits to cover the purchase of actual materials. It seems odd to pay a deposit to the contractor for both sides of the duplex, unless work was happening simultaneously (and that doesn't seem to be the case here). If they were doing one side then the other, it would make sense to only pay a small deposit for one side at a time. If their contract required you to pay a substantial amount up front for work that was weeks away, that's a big red flag.
-If you determine a contractor has done an inadequate job, notify them at the time and give them the opportunity to fix it. If you/another contractor fixes it, you're much less likely to get any sympathy from the original contractor.