Two electrical questions: Tripping GFCI, and unlocatable GFCI
Get the freezer off GFCI.  It should not be on a GFCI outlet; it's not that type of load.  It has a metal frame with all the electrical bits in an impossible-to-access location, and you're not likely to drop it in the sink.  Fit a single receptacle off-GFCI, and label it "Freezer only".   Re-task that GFCI to a circuit where protection would be helpful. 



There are at least 3 types of GFCI device: GFCI+receptacle combo devices (you know), GFCI+breaker combo devices (which look like breakers with a TEST button), and plain GFCI (which look like a GFCI+receptacle with sockets missing).  All of them can protect a downline load.  I'm glad you know that.  Do not fit GFCI receptacles at locations already protected by GFCI. Do fit "GFCI Protected" stickers at those locations right before the inspector comes. 

You're right, the downside to using receptacles/deadfronts to protect downline locations is it can be the devil to find the receptacle! Keep looking; it must be somewhere.  It's also remotely possible that pulling a heavy load on the circuit has made a wire connection at a receptacle fail, typically a backstab connection. Those are very annoying to find. 

15A receptacles are internally certified for 20A through-current, so 15A receptacles can be used on 20A circuits.  A 20A GFCI receptacle might just be that.  However, if the 20A GFCI has the "T" shaped neutral like the above pic, then it must not be used on a 15A circuit.

When an appliance trips a GFCI, that means the appliance has a ground fault. That's what GFCIs do. That's their one job.   It's amazing how people are in denial about that.  It's like if you taught them to detect steel with a magnet, and they try their toaster and it sticks, and they're all like "No way, this CAN'T be metal, there MUST be something wrong with my magnet!" And they try 3 more magnets and you're like dude... 

That's how people are with GFCI trips. 

You've performed due diligence; you tried it on other GFCIs. The answer is conclusive.  The pool pump is a human-safety device (electrical drownings) so a ground fault is 110% unacceptable, and they also have a tendency for ground faults, so it's not a surprise.  You can try a bench overhaul, but most likely into the trash it goes.  For the freezer, it's faulting too and that's common, but no way. We're not throwing away freezers for that, because it's not a threat to anyone and a lot of brand new freezers trip GFCIs. Nature of the beast. Just make sure it's solidly grounded and call it a day.  The inspector won't like it if it's a general-use receptacle; but he's unlikely to flag a dedicated freezer-only receptacle because he knows the score.