Sky-Captain, the way you mentioned trimming was as if you had it trimmed out at Flap 5, selected landing config and trimmed, and left it at that. Every power change, no matter how small on one engine, every speed change, you have to retrim it out. Don't bother with the rudder trim too much, you can't keep taking your head away from the panel. In your scan, keep checking the control wheel. Your feet should be fairly frozen on the pedals- as you increase thrust, push a bit more rudder on, as you decrease, take a bit off, but all the time in your scan, you should check the wheel horizontal. If it starts tipping, move your foot on the lower side of the control wheel. When you start going out of trim like that, the Flight Director is no longer giving you USDA Grade 'A' information!
Forget worrying about crab- that is nothing to do with it. If you are in trim, the FD will sort that out for you.
Remember, trim, trim, trim, then for good measure, trim again. You should walk out of the sim with a trembling leg!
Don't worry- you will walk it next time.