BIG-IP : expected behavior of redirects in irule stack

Once HTTP::redirect or HTTP::respond is issued, is this route immediately acted upon ? ( i.e. no further iRule processing occurs )

no, further irule will still be triggered.

Or is it possible that subsequent iRule could effect a different response ?

yes

generally what people do is either to disable further event (event disable command) or perform action on the last (lowest priority) irule (set variable in higher priority irule but execute command in the lowest one).

event