Hi Val and Rada,
on the missing Uruk Hai, they could possibly be Mauhur's lads who tried to relieve Ugluk by charging from the concealment of the forest towards Eomer's force, but were easily dealt with. Looking at the timings they couldn't be Ugluk's unit.
see here for some discussion
1st Battle of Isen on this page
I've also been a bit confused by Erkenbrand, as far as I can see he was not at either of the Battles of Isen, but was supposed to be in a position of command at the second battle. I guess its possible that he was trying to get to the Fords alone or with a few retainers when the invasion was going on, so must have had to detour to avoid Sarmuan's army.