One thing you mentioned in your details is copy/pasting, then copying a different piece of text… then wanting to be able to paste something that you had copied earlier. There’s no getting around that. Your clipboard will only hold on to the last thing you copied – it’s up to you to remember what you copied last. If you find that you need to navigate away from your Compose window, you can save your email as a Draft, then find and copy the text you need, then return to your Draft to keep working on it.
However, another approach is to add the person as a contact, so that you can call up their email address by starting to type their name in the To field. At least that will mean that you don’t have to remember or search for their address. But if it’s an address you’ll never use again, then you’re trading off the time you spend making a new contact entry vs. the time you save not having to look for their address. You’ll have to decide which is more important to you.
I find gmail can be a little touchy in terms of copy/pasting text (not just addresses). Sometimes I have to try more than once, and be more careful the second time to keep my cursor over the text I want to copy. Just try again if it isn’t working for you the first time. But copy/paste is definitely possible from the To field in the Compose window, and also from the To field in a received email.
@tom_g Double-clicking doesn’t actually select all of an email address, only the part before or after the @ sign. You have to click and drag to select the whole address.