I agree. It was fairly low on my list of things to sort out, so ended up not getting done before you and @tiffle nudged me.
Then I must have misunderstood. What do you mean by 'deselect a checked range'? In this latter version, there is no 'selection' except for the current highlighted line. ⇧↑/↓ marks adjacent items, as does clicking and dragging. At no point is more than one item selected.
I've been waiting for this moment. The big reveal... Drum roll please...
I dun told y'all!
I don't know the first thing about HTML! 
Well... that's not actually true any more, as this process has been pretty much the most comprehensive and immersive learning experience I could hope for. I can now say I know about the first two dozen things about HTML... Maybe even three!
With careful prompting and an organised mind, Bing chat has walked me through implementing every feature of this script. It doesn't always get it right, but mostly it does. At a certain point, the script became so complex that Bing would get a bit confused. Poor ol' Bing. So I subscribed to Chat GPT Plus, giving me access to GPT 4, which is far more capable.
Now, none of this is to say that a LOT of work wasn't required on my part. Countless prompts, failures, conflicts and more prompts... Quite early on, the script quickly became too long to post back into Bing Chat, so I had to learn to organise all the functions and just ask for script portions that did this or that. Conflicts are always tricky because they do require me to understand the logic of the script, even if I'm not conversant in the language of it. There's probably some redundant code in there somewhere, but right now it works, so I'm not too tempted to mess with it until something breaks.
Most macros I put together now use AI generated shell scripts to replace bloated and tiresome groups of actions with just one. The reaction to my post about Bing Chat (linked above) was fairly muted, given the revolutionary possibilities for us as automation warriors. Perhaps this example will inspire others to take the plunge into AI script creation. The sky's the limit with this stuff!
Fixed now.