a) For whatever reason, I wrote b first. Same answer sounds like it applies, though.

b) You can backtrack as much as you want in the same continent as long as (1) you only make a stopover in a city once, and (2) you haven't reached your max segments or stops.

c) As quebec said, you need the 4th segment to get back to BUD, so it is the one place it is allowed. I think that technically, Budapest is considered the "3rd" stopover.

Good luck. I am still fighting a couple of things with them, and it's been about a month.