3. why do you want a shortcut for adjusting a frame to the guides?
while i'm mostly comfortable with using the mouse for it, your question has made me think of ways to profit from such a feature. but, probably, not in the way you were thinking of.
but, again, in order to know, if you indeed found a missing feature, we should again know the use cases you're thinking of...
also, i'd like to point you to the fact that when creating new frames, you can shift-clicking to get the frame to match the guides surrounding it... no need for a command to adjust the size in that (very common) case.
and here what i was thinking of: extending the arrow keys handling to enable jumping to the next guide...
probably, we can't do it by adding a modifier (shift, ctrl, alt... all are already in use) and i wonder if and how we could add a "mode".
if it's really easy to use, i can think of using it... but it should have a mostly perfect interaction...
and if we can also use modes to replace the usage of the alt modifier, than we would have a real improvement in scribus.
but it really needs to be fast and comfortable to trigger.
does anybody have an idea how we could add such modes? (user interaction, user interface)