Slippery liquids: De-icing fluid comes to mind, especially the newer types which are designed to come off during the TO roll. I can't see them evaporating very quickly at -4C, though, so heavy contamination would have been noted by the investigators. If it was really causing problems, I'd have thought other aircraft might have noticed something?
You can get a build up of de-icer on the runway surface but my experience of KDEN in the winter is that the dewpoints are so low that de-icing is not normally needed unless there has been some sort of frozen/freezing precipitation - don't think there was during the day of the accident.
Brakes: I'm not sure about the specific fit on the CO 737-500 or their SOPs but RTO autobrake is normally triggered by closing the thrust levers above 85kt(?), which should also deploy all the spoiler panels. This leaves you free to select reverse (or have someone do it for you) and concentrate on keeping the aircraft on the runway. It's possible to brake manually using either or both pedals while power is still on - don't know what that does to the autobrake logic (if anything) when you do close the taps. You can knock out the AB after engagement but you have stand on them pretty hard to do it.
Rudder: A 'hardover' would have taken the aircraft off the runway much more rapidly, probably ending up in a groundloop. 'Loss of authority' sounds better. If you just lost movement on the control surface, a 25-30kt crosswind would probably gently twist you off the runway... Just thinking out loud.
