Hi Peter. Glad my ramblings might prove of some use, at least! I'll answer your last point first, as it's simpler to address:
why do I prefer desktop view on a mobile? that's an interesting question to ask. Various 'gut reaction' replies for you:
First off - I'm used to it! And one does tend to stick with what one knows.
But, secondly, desktop lets me 'see' the whole of a talk page at once. It gives me an overview, allowing me to determine at a glance, not only the topics, but also the number and depth of replies a user has received. Whilst mobile view gives me a nice tidy page and big text to look at, it gives me absolutely no idea of whether any given topic has been answered by anyone. They have to be uncollapsed to see if anyone has responded. The collapsed topics are very widely spaced, meaning I sometimes have to swipe down quite a lot just to see all the topic headers. This applied not only at normal user talk places, but also at places like the Teahouse. At the latter, seeing if anyone else has replied is really important. Uncollapsing one topic at a time at the Teahouse would be a bit of a pain, and automatically having all threads uncollapsed would make the entire page too unwieldy to navigate through. Equally, when I go to a new user page to leave a comment or warning, it's helpful to asses at a glance if someone else has already raised the same issue without having to expand anything. (I’m not sure if showing a ‘reply count’ alongside each collapsed topic header -or some colour difference between topics with and without replies - would actually make things any different.)
A third reason I don't choose 'mobile view' for editing on my mobile is that, having opened up a topic I want to view, and then having scrolled down to the bottom of it, there is no quick and easy way to collapse the thread again. So I have to scroll/swipe all the way up again. collapse the thread, then repeat the process with the next thread and so on. I would suggest that a ''collapse thread' button at the bottom of every mobile view topic would help a fair bit there. If you go to my user talk page at en-wiki - and also to the Teahouse - you'll see I have the 'skip to top and bottom' template with its floating up/down icons enabled. Now, I really like this a lot (both when using a keyboard/computer, and on a mobile phone or tablet. The clickable up/down arrows that this template produces is, admittedly, rather tiny in mobile-desktop view, but they still help intra-page navigation. I've just tested it again in mobile view and noticed those up/down icons only appear if I click 'read as wiki' (something I don't think I've ever noticed or clicked before - sorry). I have had advanced settings enabled for some time now, but cannot get on with 'automatically expand all threads' as this seems to make an active page even harder to move around in.
Of course, I recognise it's possible my lack of familiarity with using mobile view on a mobile phone might mean I've missed a some helpful options which might have made some of the above comments redundant. So do tell me if I have overlooked some trick or other.
Oh, another thing I'd like to see improved is greater visibility of the 'mobile view'-'desktop view' link at the bottom of the page. It really is too tiny - and far too important a link to have so small. I recognise that in 'mobile view' the link is reasonable size, but in desktop view it's too 'out of the way'. Ironically the floating 'skip to top and bottom' icons also happen to position themselves over the lower right of the page, so making the link hard to click. I realise, of course, this as an optional template I've added. I would urge something akin to it to be provided as an option in User Preferences and which can be deployed by an individual user and which works on all pages, and not simply by the manager of one particular page).
Now, you asked about 'replying' at the Teahouse, and you offered a possible 'workflow' I might have been using. It wasn't quite the one you describe. What I currently do in desktop view on a PC/laptop/tablet or iPhone when I want to reply to an editor and ensure a ping is:
1 Click 'edit source' after their newly-added topic
2 Use mouse (or fingertip on phone) to select username of the OP. (Impossible if they've forgotten to sign, of course, and if SignBot hasn't done so for them)
3 Indent my reply with a colon, and (using either ping, re or u, depending on circumstance) type two curly braces, letter 'U' followed by a pipe symbol, then paste in the copied username, and close with two more braces (i.e. {{u|PPelberg (WMF)}}) and then continue typing my reply. This is so much easier to do with a keyboard than on a tablet which, in turn, is easier than on a tiny phone screen. I rarely use the format [[User:PPelberg (WMF)}]]. If I were King of Wikipedia, I would provide everyone with a Preference option to warn them if they've tried to Publish Changes to a user talk page without including their signature. i.e. much in the same way as one can choose on en-wiki to be warned if one forgets to include an edit summary. (Which reminds me - that function doesn't appear to work in mobile view, which surprised me.)
Finally, yes, the link you provided was indeed the comment I was referring to. Great to hear these will be addressed soon. I have, over the years, collated a few other suggestions for things I'd like to see improved. You might find one or two other things of relevance at https://en.wikipedia.org/wiki/User:Nick_Moyes#Feedback_and_Suggestions.
Goodness - that became a long reply. Hopefully some of it will make some sense. Should you find it helpful, I'd be happy to address any other questions (a.k.a. ramble pointlessly at great length!) via Skype or Zoom. Just get in touch if you do. Cheers, Nick