I often visit Mill Creek Falls and Pearsoney Falls, but have not done so this late in the season. This region, between Prospect and Crater Lake and Diamond Lake, gets quite a bit of snow most years. Highway 62 and 138, which lead from Prospect and around Diamond Lake, will be plowed and remain open in almost all cases. You’ll want four-wheel drive or snow tires if conditions warrant them. The smaller access roads to Barr Creek Falls, Salt Creek Falls and Diamond Creek Falls may well close.
I think a winter trip to this area is a very fun idea (we spend time in this area during the winter ourselves), but if you go, plan on the chance of snow, and perhaps rethink the trip to being a snowshoe trek or snow play type of experience. Instead of planning to hike to waterfalls, I’d plan to snowshoe or cross country ski in the Diamond Lake area or at Crater Lake. Diamond Lake’s resort is on the rustic/bare bones side, and Crater Lake is historical and more upscale. Snowshoeing around the lake is stunning! Of course, during the time of your visit, the snow may be gone, in which case you could do the same routes on foot.
Either way, you’ll have a fun trip, but flexibility will be the name of the game!