In 2019 I don’t remember it raining all that much but the last half of the season was a mudfest.
I’d walk the course, not ride it. Unless you have multiple bikes and/or the ability to clean them doing a few laps just makes thing worse quicker. As long as you know what’s coming up I never found that pre-riding made a huge difference over just looking at the course. Besides in a muddy race the lines can change quickly as ruts form and grass disappears.
Warm up wise I didn’t use rollers. If you don’t have a van then I saw people using tarps at the side of the car. If it’s not windy you can put a big brolly in a workstand.
My approach was to take waterproofs and ride on the road. Some cheap cut down trousers and a jacket you can wear to warm up and get to the start grid. Throw over the tape with a minute or so to go. If it is cold then spare gloves are a good idea.