1. It's possible. But also could be appearing naturally.
2. Probably not. You may need to show us the SMART report.
3. Isolated cases are fine. If numbers are still growing, that's a bad sign.
Working at higher performance will generate more noise.
Your understanding is incorrect. Only DMSMR drives are not very good because the disk is managing zones like a black box. HMSMR are great because user have full control on how to write data, thus eliminating all the shortcomings.
I would no brainer go to a new exos.
You asks if you have to increase budget, but you didn't mention how much storage space you are looking for. New drives are at 18TB $199 price point, historical low. Unless you want to buy small drives but I don't think you'll get the most out of your money.
At this point I'd probably just go to https://www.reddit.com/r/buildapcsales/comments/17xmwu7/hdd_best_buy_wd_easystore_18tb_external_usb_30/ .
Otherwise, refurbished Exos 16/18TB if you are not a fan of shucking or WD.