1/ No idea, I don't use Qt Creator. Assuming it's simply gdb over gdbserver in disguise, probably, as I ship gdbserver. Not my cup of tea, though, I much prefer doing it natively, so I've never really looked into the remote approach. We may have some rudimentary documentation about it in the KOReader wiki, though.
3/ Yup, that'd be the QPA you want. Except it may need some tinkering to deal with the Libra properly.
3b/ Keyword being "may"
. The reasoning being it provides a battle-tested implementation of everything you need (i.e., a working UI toolkit) and is fast to prototype and iterate for, because Lua. And will handle any and all low-level device quirks for you, and there are many of those on kobo
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