Hey BCB, I have never fully disassembled a mixer (so have hestitated to comment), but it sounds like you could have clutch washers that need to be changed. I know many of the other bingo folks on this forum have done so, I'll try to direct them over here. I have only disassembled down to the problem mixer then stopped.

My friend and bingo mentor explained his process one day - as you disassemble, put the items on a wooden dowel with a piece of paper between each assembly, labeling it.

When you go to reassemble, you start at the top of the dowel and work your way down.

Straightforward and easy to do.

The shafts of the mixers can get really cruddy, too.

Quick question for you - are you using Neatsfoot compound or neatsfoot oil? You should be using the compound, not the pure oil on the clutch washers. It's in a red/white/blue container (at least on the East Coast).

Key West and Miami Beach are really fun. It's great to hear how much love you're giving those games.

the spacers between the bits on the shaft are probably plastic, so you don't need any lube on the shaft or those pieces. If it dries, you can get continuous squeals. The score extra step assembly and the spacer tube between it and the spotting ratchet is metal. You can wiggle/push the parts with your finger and see if the squeal changes. 

look at see if each mixer latch release arm roller is constantly riding on the cam edge, and if so, is the roller spinning.  You can see if the roller is causing a problem by pushing the top of the release arm where it pokes through the top plate towards the door - that'll move the roller off the cam edge.

fwiw, the motors can be quiet on the bench and loud in the machine due to the load on the gears once installed. If you want to check on the bench, stick something thru the shaft hole and put some resistance on the spinning.

what kind of 90W oil did you put in the gearbox? No troubles with it weeping out?

if you do take the assembly apart, the main thing to pay attention to (besides the order of the bits) is the orientation of the thin metal latch arm release cams. The cam lobes will line up differently if you flip them around ... it may not really matter - it would just change the timing of when the rotors latch relative to each other. If things do get muddled, you'd want to mount the cams to offset the lobes as much as possible between adjacent cams.  i.e. you want adjacent rotors to latch at different times.

if you do take the thing apart, can you take pictures of the rotors and both sides of each mixer contact plate? The goal is to make the mixer diagrams, which aren't in the manual.

also, if you're in there and can take/post a picture of the reflex unit wipers with the wipers reset, that would be good too. If there aren't rivets under the wiper fingers and it's not obvious in the picture, please note it. 

you need to rebuild the mixer.
lay the back door flat.
pay close attention to the drag-links assembly or you'll cuss a lot later,
place each removed item on a wire or dowel rod to keep them in proper order/orientation.
use a piece of paper to represent the mixer discs when you get to them.
clean up each item when removed (or when reassembling as I do).
leather clutches can be cleaned (dawn & h2o) dried an re-oiled if not badly worn/torn.
notice the uneven driveshaft ... that was smooth and straight when new
it'll take a full afternoon.