I don't know with postfix for point 3 and 4, but we use Haraka as incoming and outgoing gateways, and you can do almost anything you want as you can write your own plugins using hooks.
I wrote a plugin that detects it was a forward from SM to apply some rules to them. It also could be used to force using another gateway.
Well it was a bit tricky but I based the detection on the From enveloppe address having a SRS pattern (only works when SRS is enabled though).
Then I can decide what to do with it, including relaying it through another gateway.
That was an old trick and I think now SmarterMail adds some Headers such as, X-OriginalSender, X-ForwardingAddress which could maybe be used for identifying forwards.
EDIT: It was added in build 8747
Added: Email headers for X-ForwardingAddress and X-OriginalSender to help the tracking of automated forwards.
But still, of course having this routing flexibility integrated in SM would be nice :)