Ok my opinion and experience here (sorry a bit longer post):
in first place marshall has to be sure that his action won't interfere other cars - thus checking if there are cars coming before he runs for a car, than again same check before he's going to put it back on the track. Oh and of course again same check when he's on the way back to marshalling position.
To order of marshalled cars - this one is personal
I marshall them in order they crashed if they are close to each other and I've seen order of crashed cars. If not I'm just marshalling cars based on distance from me.
It can be very frustrating if there is case of someone crashing into you and sending you both to track marking or you doing mistake first and guy behind you end does the same mistake and ends up in you so you both need to be marshalled and if the other guy gets marshalled first. That happened to me several times.
Anyway I strongly disapprove 2 things - shouting from rostrum on marshalls (it's 99% your mistake you shouldn't do and marshall is only human - I don't even shout if they don't notice me - but that's me). If someone heard me shouting from rostrum at The Players let me know
I had to be marshalled there several times
Other thing is full throttle before car is put back on the ground - that can be really annoying and I punish that by not putting car back on the ground until he stop revving his car. Marshalls can get injured by that.