Wondering if "Lower Resistance" spells stack on top of each other. If I encountered a situation where I knew that the enemy had rather high spell resitance, could I cast multiple spells to lower them down?

Yes, they are cumulative. But take care, because its duration is not too long.

Just about any spell that pulls some kind of percentage is cumulative. Pierce Magic, Breech. The one's that boost something by a fixed amount don't usually stack, but the percentages do.

Also, I don't remember using LwrRes a lot, but I would guess it's duration increases with level.