@Joker
Short answer: Success!
Longer answer: Didn't quite get how I'd remove the warning, rather than the torrent - which I'd done on numerous occasions. Won't bore you with what I actually did, but...
I decided to follow @ianfontinell 's advice, and to upgrade my qBittorrent client beyond the 4.6.3 version available in the Linux repository. I installed 5.2.1 as an AppImage. Once I had this running, I added the "Warning" torrents (deleted from the previous qB instance before upgrading), using freshly downloaded .torrent files found by searching the site "from scratch", rather than clicking previously used links, etc, so that everything was as unencumbered with baggage as possible. Forced recheck and started... no warning. Yay!
At the same time, this seems to have resolved another recent - and actually more important - problem. I tweaked my settings, following the announcement about IPv6. I think I set it to "IPv4 only", but I'm not absolutely certain. Anyway, since then - or possibly before, and that's why I tried a tweak - I'd noticed a drop in how much I was uploading. Not a huge amount, but I was watching my ratio slowly dropping. I normally manage to maintain it, or even improve it gradually if I set my mind to it. With this new iteration of qB it now appears to be running at speeds similar to what it used to. Early days, but extremely encouraging!
My thanks to you, @Joker, for all your continued help on this. And to you, @ianfontinell, for persisting in your advice to improve my client until it finally got through my thick skull!