Second questions: Lets assume the mob they had via /assit is dying and I forget to give them a new target in time. Is there a way to let them attack my focustarget automatically in case the assist-target dies as long as I do not give them a specific /assist target again?
Well I do this with an ftl, but in general it should also work with an focussystem like this:

1. Create a macro like
/stopmacro [harm,nodead]
/assist focus

Put it on lets say 'AcctionButton10'

2. use a click to that Actionbutton in your dps-spells like:

/click ActionButton10
/cast Fireball

this way, if you have selected a harmfull,nodead target the '/click ActionButton10' does nothing (becasue of /stopmacro [harm,nodead]), but if you haven't a target, your target is friendly or your target is harmfull and dead, the target is updated with the current target of the leader.

..i think the same approach could maybe solve your initial problem.