For (a) I've been using rofi. I bound a key to open its window switcher which gives me a searchable list of all open windows. There are quite a few other options: https://wiki.archlinux.org/title/I3#Jump_to_open_window

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.