OK. I have the solution to my problem with 2 Apple IDs. And it's not simple. There are 2 answers to the question of how to merge Apple IDs that are the same except one is @mac.com and one is @icloud.com:
1) You must delete the iCloud account and ID and wait 30 days, after which Apple can do what they were supposed to do in the first place and create an iCloud account for you based on your Mac account. There are 2 problems:
A) You will lose everything in your @icloud account unless you figure a way to manually save it all off
B) You run the risk of someone else grabbing your username for their own iCloud account while you patiently wait to get yours back
2) The almost 100% good workaround is this:
A) As your @mac.com personna, turn on Family Sharing and invite your @icloud.com personna
B) As your @icloud self, accept your doppelgänger's invitation
C) On one of your devices where you've always been signed in with your @mac ID, backup everything locally to iTunes on your Mac (where you're also signed in as .mac. Note that you're in this situation because you had your .mac ID first before iCloud arrived and Apple didn't move stuff to a new equivalent iCloud account for you properly)
D) Sign out of your device, agreeing to turn off Find My [device}, and
E) Say YES when asked if you want to keep a copy of the data on the device (your @mac ID's data)
F) Log back in with your iCloud ID (mine's the same user name but I guess it doesn't have to be)
G) As you log in it will ask you if you want to Merge what's on the device with your @icloud account. Answer YES INDEED!
At this point you have basically overcome Apple's limitation. Do this on all your devices and you will have all the everything that your old @mac.com ID had now available to your @icloud.com ID. Cool, huh?! One caveat - it appears that if you recorded musical ideas in Music Memos, they do not come over when you log into your device as your @icloud self. This may be a bug in the Merge function. :-( Clearly Apple doesn't have these capabilities working completely. The process isn't absolutely perfect so it might be a good idea to first backup your device with your original @mac.com ID to *both* iTunes and as much as you can to iCloud.