The floating button is currently being overhauled which I think should make changing the position easier even if it doesn't support it out of the gate.
In the mean time, you could turn off the floating button for the feed view and instead use the feed button on the bottom navigation bar to scroll back to top. Bit of a hidden functionality :) Don't think there is currently a work around for the post view though unfortunately.
I really like the idea of custom feeds made up of selected subscription groupings. Can't speak to if or when such a feature would get implemented. But regardless, if you have the time to open an enhancement issue on GitHub, we would greatly appreciate it!