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I wonder if there is any connection between exposure to high moisture levels/ambient humidity and this issue? Might be worth taking extra precautions... storing in case with dry silica gel and trying to keep as dry as possible?

Precisely! And there is no phantom power on those, only on the "surrounding" XLR.... So, you have 20 line inputs on TRS if you need them, not 4.

If you are connecting external preamps USE THE TRS JACKS. Phantom power does not appear on these. If your external pre has an XLR output socket, simply use an XLR-TRS adapter cable.

Yes. There really is no problem with this. The only time there would be a problem is with certain older model ribbon microphones, or microphones that are wired incorrectly or have defective cables. Modern dynamic microphones using balanced connectors are safe. You also mention connecting "preamps" - you would not connect these to the XLR's, but into the line-level TRS Jack connectors. These do not carry phantom power. Phantom power is seen only on the 3-pin XLR connections. I have been running phantom power through balanced XLR's to a wide range of dynamic microphones for many, many years. Not one problem, ever.