aah it's making sense now... firstmiddlelast *is* a me.com/mac.com address because of the alias factor. also you must have verified the address via that channel.
bogus about the alias non delete, because if you could convert to a family 'member' vs alias that would get what you want including that address can go to the cloud.
you can't make an @me same as a different @mac they are the same.
unfortunately alias transfer as aliases not sub-accounts, so as you realized, can't convert it to a real account afterwards.
Ok, so the tagging solution is your best bet and probably better than having a separate email anyhow.
on your firstmiddlelast@mac.com apple id, you can change your primary email to:
firstlast+itunes@mac.com
it will send verification to firstlast@mac.com so you can lock it in; that at least eliminates having another email (gmail) involved it also has the bonus of sending your purchase receipts to your main email.
think of like in my case:
my iTunes applid is 'firstmiddle'
my primary email with that ID is FM+itunes@name.biz
my iCloud appleid is firstmiddle@me.com
my REAL email is FM@name.biz and that is also my primary email; i ERASED my primary email of firstmiddle@me.com i never use that anyhow. I did that so my find-my-friends would work with my non-me email.
so in your case:
FirstMLast@mac.com apple id would get a primary email of FirstLast+iTunes@mac.com
you'd continue to use FirstLast@mac.com for everything besides purchases
for purchases you'd use FirstMLast@mac.com but purchase receipts will go to FirstLast@mac.com
i don't think you'll get a better solution than that. it's actually good to have purchases split from regular iCloud biz anyhow especially since you can have separate passwords; let the boys have access to iCloud services w/ot cart blanc buying iTunes for example.
ps you are 'already screwed' not being able to directly re-assign your apple id to your email, also i tried to create a new apple id with a previous alias and that fails so you're out-of-luck with that line of reasoning as well.
even IF you could delete the alias and re-create a sub-account, it won't let you add an email that has ever been an apple id, so i think STOOPIDLY it wouldn't even let you assign that email address else you could do it right now. I think more than likely you'd get stuck at he creation point because you would not be allowed to make an email address which already exists as an apple ID.
my solution will let you combine both into one from a functional standpoint while keeping the actual purchase login different. (you only have to set that ONCE per iDevice anyhow, and if you've already been using it then just keep usin' it).
The ONLY other reasonable option: bail on the FirstMLast@ and make your purchases with FirstLast@ ; you will have 'old purchases' and 'new purchases' so when doing app updates you'll have to put in passwords twice depending if old or new are updated. i recommend against it. That said:
if your purchases are MOSTLY with @earthlink and only a few are with the mutant FirstMLast@mac, than it would be a decent solution to bail on that one and either create a NEW appleid for purchase (say for example: FirstLast+itunes@mac.com) that would get you a CLEAN solution as well.
your purchases will all be tagged with the primary email address at the time of purchases. you can QUICKLY determine exactly which songs/movies/videos were bought under which email using ths amazing script i found online: http://dougscripts.com/449
(it originally only searched by 'real name' which typically stays the same; i wrote in a suggestion to the original creator and he added in the OPTION of either name or account).
run that applescript set to 'account' on all your 'purchased' or 'protected' tracks and you'll quickly get a set of playlists containing all the songs bought by EACH email account.
in my case I had ELEVEN but 3 of the accountd had less than 3 songs so i converted all the 'small count' ones to MP3 by burning to CD and re-importing, now i'm down from 11 to 4 accounts.
determine based on which has the fewest which route to take.
the accountname will stay the same (FirstMLast@mac.com) in any event so that one is here to stay, meaning i'm back to recommending that you for convienence sake use whichever email you use the most and set that as your primary email in the FirstMLast@mac.com appleid; either youremail+itunes@gmail.com or FirstLast+itunes@mac.com from that point on at least your receipts will go to the same place. you'll still need to 'add the M' to login for purchases but you do NOT have to add in the +itunes that's not the login, that's just for delivery.
IF on the otherhand, you have VERY FEW purchases with FirstMLast and most are earthlink which you can not switch back to anyhow. (although it would be worth a SHOT); hmm be right back.
the thought was this: re-create a NEW apple ID with your EARTHLINK or GMAIL address from scratch.
if you look at your purchase history (using the script above) you'll probably have most purchases there. I think you said that email is bunk now anyhow, so you will have SPLIT purchases no matter what you do.
if you have 'more than a few' with the mutant id, than it's best to go with plan-a; FirstMLast+itunes@mac.com as primary email. If however you'd like to break free from the goof and have an appleid that maches the email, you could as you suggested, create a brand new 'firstmiddlelast@me.com' or just use your gmail address (that's guaranteed to be available); youremail@gmail.com as a brand-new appleid.
let the old one die; if you are a purest and only have a few purchases with FirstMLast you could re-purchase or delete 'em and still at least only have TWO iTunes accounts in your library.
so, i'm interested to see what you'd decide to do; it's pretty harmless to try out the tagged email solution since you already are past the point of no return; you can re-change the primary email as often as you'd like (just beware when using @me addresses; it will preculde you from using that as an apple id standalone in the future).
-awr