Each line in the macro has to be homogeneous wrt the effect. IOW, you can't either cast hunters mark or assist because the root of the command is either "/cast" or "/assist", etc.

I also don't think there is any way to make the kind of decision you are looking for. I wish there was, cause I'd like to do a similar thing with party leader.

Assuming the non-focus party members don't have a target, and assuming the focus has itself as focus (is that even possible), you might be able to do something like:

/cast [harm] Hunter's Mark
/target focustarget

So the cast won't happen for anybody but the focus (since they don't have a target), and everybody will target the focus target (including the focus).

I'm sure you know this, but you can also get away from having to "/target focustarget" by just putting [target=focustarget] in your attack/cast macros. If you do this, and if you are running keyclone, you can just put hunter's mark on the 3 key on each client and put "3" in the do-not-pass list. I do something similar in a couple of places in my setup.