I have a at least a couple of hundred identical entries in my Keychain that are:

Name: iMessage Encryption Key
Kind: private key
Keychain: login

There are no Modification or Expiry dates. Two questions: 1) What are these? 2) Can I safely ace the whole works?

I have an equal amount of iMessage Encryption Keys and iMessage Signing Keys in my Keychain.
I’m sure they are part of what makes iMessage work securely, encrypting each message, as it travels the hostile Net.
As the keychain may be shared with iOS devices, these keys could have nothing to do with your Mac (if you haven’t set up Messages for macOS), but deleting them on your Mac could break access to (past) iMessage conversations on your iPhone or iPad. There may be no fallback mechanism for decrypting old messages without these keys.

Okay....good to know, thanks. Since I only discovered them by accident, and was otherwise oblivious to their presence, I'll just leave them be. Better to do nothing and be safe rather than do something and be sorry.