Well, assuming it wasn't a different project (projects have their own indent settings), that file must have confused Geany's detection by having some lines that start with tabs. If you provide the code it might be clearer why.
It was Python 3.0 stdlib. I doubt, if there were any tabs, because this would confuse the intepreter quite a bit ;-)
No, I think this would be too messy to implement. But we could make the Spaces indent mode a bit easier to use, perhaps a pref to make the <- -> arrow keys jump through the number of spaces of the indent size.
I would rather have backspace deleting a tab length of spaces. Together with tabs changing to spaces, it would save me a lot of trouble.