Belated response: yes, the resynced wallet eventually caught up with my transaction history, and thus reported zero balance. Although interestingly when I loaded the next most recent wallet backup (created a month after the original file mentioned in OP) it began showing an "available balance" exactly the same as the ~0.5BTC sum mentioned earlier in this thread.

Any attempts to perform a test transaction to move a fraction of that amount show the same "0/unconfirmed, not in memory pool" message that I reported earlier in this thread.

So for all intents and purposes, the original wallet has been emptied. I've been running through various other old wallet.dat files backed up more recently than the original file, and they're all empty too. Looks like I'm coming to the end of my historical treasure hunt... although one file (created in late 2015) is showing a message when I try to load it which ends: "...wallet is corrupt. Try using the wallet tool bitcoin-wallet to salvage or restoring a backup."

I've done a little research around that error message and will soon try a few things to investigate further. I doubt it'll result in any further discoveries, but of course I want to be thorough here. Feel free to make any suggestions re that particular error message if you're familiar with it. And thanks for taking the time to read and reply to this random thread.