Ok, firstly an update: Since I last wrote, “paste” has started working. Couldn’t paste yesterday, can today; go figure. (I have no explanation for that, as I changed nothing.)
Also, “edit in-place with app” is now working. Basically, the working / not working stuff is now like this:
Can edit text files on phone with an app (eg, Kate).
Can “copy” files from phone to computer.
Can “paste” to phone from computer to phone. (Yay!!! This was the main thing I wanted to do!)
Can “delete” files on phone.
Can’t “cut” files from phone. (Workaround: copy file from phone, paste to computer, delete file from phone.)
Can’t “rename” files on phone. (Workaround: copy file from phone, paste to computer, rename on computer, cut file from computer, paste file to phone, delete original file from phone.)
Can’t create new files on phone. (Workaround: Create new file on computer and copy to phone.)
Yep, that makes sense. Mirrors the restrictions on the “C:\” directory in Windows 8 & 10: Can’t create files there in-situ, but can copy them there from elsewhere.
Btw, my previous statement “it’s not a permissions problem” is maybe not true. When I enabled the “owner” and “permissions” columns in Dolphin, the “owner” of all files on my phone is [blank] (I don’t even have permission to see who the owner is, apparently), the directory permission are all “drwxrwxrwx”, and the file permissions are all “-r-xr-xr-x” (no write permissions at all!). This is kinda backwards from what I’d expect based on my can/can’t list above. Weird.
I’m guessing that Windows Explorer’s implementation of MTP just automates the “workarounds” I list above, whereas Manjaro-Plasma’s Dolphin doesn’t.
Btw, I verified that my spare notebook computer, running a pristine install of Manjaro-Plasma (installed just a few days ago) is doing all the exact same things. So this isn’t some fluke on my desktop PC; it’s just how Manjaro-Plasma’s Dolphin interacts with Samsung Galaxy S7 phones. I suspect it’s a Dolphin thing.
Of course that doesn’t quite explain why the three other MTP apps I tried failed to work. But that may be some combination of me using them wrong (their documentation ranges from “abysmal” to “non-existent”) and KDE’s own implementation of MTP interfering (as hinted by that one app that said “Error: MTP already in-use, perhaps by KDE or GVFS”).
But I now have workarounds! So I can now copy stuff between phone and computer. Just, no renaming-in-place and no creating-in-place.