Re: Diving into an old BTC wallet from ~2014 and found a nice surprise - what next?
Interesting, I was certain that Bitcoin Core displays an error/warning if you load an old wallet into a "pruned" node warning that it may not be displaying the correct history/balance etc... although I might be confusing it with the warning you get if you "unload" a wallet. 
Definitely no warning shown, and I downloaded the newest version of Bitcoin Core available. Might be an oversight from the developers, or maybe I'm the first idiot to have come across this self-inflicted error!Blockchain is at 87% synced, with ~37,000 blocks remaining, and it should be done by this time tomorrow. Maybe.
To your other point: the wallet is showing an available balance of zero in the background, while resyncing.