... the file I wanted to transfer, a Contacts archive file from the newer to the older iMac would not open!
and
That every file I've got on my Monterey iMac might just be unaccessible when I make the leap to the M chip?
... are two separate concepts. As far as macOS and other Apple programs go, it is normal and expected for upgrades to migrate files from older to newer versions relatively seamlessly. It has been that way since forever. Every Mac that I have ever bought has inherited the contents of the Macs they replaced, even to the point old printer drivers that can no longer be installed (for example) continue to function.
On the other hand expecting anything to be backwards-compatible is not advisable and almost never works. A Time Machine backup created with Monterey (for another example) may or may not be usable or even readable by Mojave. It was never intended to be used that way.