Changing iTunes ID email and previous media downloads

iTunes ID created for music purchases on a PC long before I had a Mac.

So I use separate iCloud and iTunes/Mac App Store IDs on Macs and iPhones/iPads.

Until 2 years ago I got emails to a linked iCloud email address advising of iTunes purchases/subscriptions etc, they abruptly stopped. I do not know why.

The iTunes/Mac App Store ID is a defunct email address and when I try to manage it I am asked to verify it. I'm reluctant to try to do this as if it goes to the defunct email address I naturally won't get the email, and having not received emails to the previously linked iCloud email address for 2 years I worry that has been 'unlinked' somehow.

I can log in using the iTunes/MAS AppleID but I want to manage the AppleID (defunct email address).

I've purchased a heck of a lot of stuff from iTunes Store and the MAS over the years and have generally always downloaded TV Shows/Movies to have a local copy for use via Home Sharing with AppleTVs.

So:

1) If I try to verify with the pre-populated defunct email will verification go to that or the one I used to get invoices etc to?

2) Assuming I don't receive a verification email will I get locked out?

3) Ideally I'd change the email associated with the defunct email used as an ID, however, while I appreciate I will be able to redownload purchases if they're still in the store:

a) Will previously downloaded movies/TV Shows/apps (some many years ago) still be playable/functional?

This is important as several old purchases are either no longer in the store or unavailable to redownload.

As I have older downloads will these now missing purchases still be playable?

b) Will any subscriptions or one off 'unlock purchases' for apps be lost?

I'm struggling to find the answers to these questions from searches, other queries and even from Apple support who can give no guarantees, and can't even assure me I'd be able to recover to where I was if things messed up! All they can advise is to change the email, it may or may not work, not their problem, and they can't escalate until I try a potentially irreversible step which could lose me access to many previously downloaded purchased items.