We have added an undocumented "kill" keyword to stop processing:
if (foo) then
You cannot (yet) direct arrows in other direction.
However our plan is to implement an "horizontal" layout.
(Today, the only possible layout is "vertical").
This future layout will allow to lay activity diagram from left to right (or from right to left).
Once this will be done, then yet another layout will allow to mix vertical/horizontal layouts.
But don't expect all these before several months...
Anyway, thanks for your suggestions, and others idea are welcomed!