For (b) I don't know what the best option is but I see some people posting their scripts in this thread: https://github.com/i3/i3/issues/838 I think I would bind some keys to mark and then focus by mark if I wanted to do this instead of having a toggle.
Not for windows as far as I know, but for workspaces, yes: https://i3wm.org/docs/userguide.html#back_and_forth https://i3wm.org/docs/userguide.html#workspace_auto_back_and...
But grouping windows logically based on how they're used has always just felt right, so I've never really considered that you want to keep track of the order in which windows were previously selected (using alt-tab to navigate even just three or four windows can be quite a trick). It always seemed like a necessary evil since floating-only window managers can't handle the kind of spatial focus-navigation that i3 does.
Alt tab with a long hold on alt lets you select another window, albeit from a linear list as you describe, by cycling through with tab and shift-tab.
Quickly typing alt-tab now cycles between the window you came from and the window you just selected. That’s the super useful value of the feature.
Is there an i3 command to (a) leap to another window from a selection and (b) leap back and forth between the window you came from and the window you just chose?