The first thing is proper identification. Red Thread and Pink Patch generally occur in spring and fall, not winter. Red thread has antler like projections called sclerotia that grow from the blades, so it's easy to tell if you have that. I suspect you have pink snow mold or Fusarium patch. What was this winterizing treatment you used to prevent snow mold? Did you mow your grass lower than usual once it was dormant, or did you leave it long? Most of the winter molds only affect the blades, leaving the crowns intact and will clear up once growth resumes in spring.

There is no way a pre-emergent herbicide can distinguish between weeds and what we may consider non-weeds. They are simply all broadleafed plants and the effect should be the same. That's why it is not recommended to use pre-emergents where you will be sowing seeds or allowing self-sown seeds to germinate. I would attempt using Preen or some other pre-emergent before I'd resort to RoundUp, even in clear areas under the tree canopy. There is some evidence that repeated use of RoundUp across a tree's root system can have damaging effects. I realize you may be dealing with a great many seedlings, but I'd recommend manual removal over and above anything else. I deal with freely seeding maples myself and understand what a PITA it is to get rid of them. Raking up the samaras when they fall would be my first step. The less of these that remain on the ground, the less germination that will take place and many fewer seedlings to deal with.

It's possible. Can you bring the camera up to 3 inches away? Take the picture in the shade or on a cloudy day with no flash. What is your watering regimen? How often and for how long do you water? Or how often have you been getting rain?

We'd need photos, but usually red thread doesn't kill the lawn and treating it past feeding the lawn isn't worth the time and effort. It'll fix itself when conditions improve and either the rain stops or temperatures warm or cool outside its viable range (warm, in this case!) If it's really, really bad and is doing damage, which is unlikely, you can use a fungicide containing benomyl or carbendazim (per Wikipedia, anyway). I've never bothered to treat the small amounts I sometimes get during wet spring and fall weather, so I can't say how well or even if that would work. Please note that both of those fungicides have some really severe side effects on other populations (worms and lab animals when tested, respectively) so you'd want to think long and hard about use before pulling the trigger. Personally, I'd pass and wait for the weather cycle to end before doing anything.