Hey there. A good deal of your questions have already been answered here in some form or another. Long story short, in my experience the population is fairly healthy on horde side. Whenever I was looking for a group, I usually found one within 15 minutes of searching. Depends heavily on the time you’re playing on, of course.
If you’re looking for guilds, Thalei kindly put in the work to create a great Guild Index which might help you find something in that regard. I’ve seen a decent amount of people in T-sets, so I’d say that yeah, there are guilds clearing MC already.