Hi ricecakes,
I'm glad to answer your questions:
1) My muscles did not spasm. They would feel very tight though, and when I would massage sometimes they would feel unusually hard and sometimes like a rope. I would also find a lot of muscle knots. I'm not sure how significant it was though, as I still get muscle knots/hard spots and they don't bother me. And although the muscles felt tight, it didn't seem to limit my range of motion past what is normal.
2) Yes I experienced hot, burning sensations. It's hard to say if it was precisely in the SI joint, but I know it was in the muscles around the area. I don't recall numbness, but I would get tingling sensations along with pain in my butt muscles.
Once I started feeling better I stopped trying to correct my SI joint alignment. It may still be crooked, but I haven't checked. Maybe squats and deadlifts straightened it? I bet it is still crooked though, because when I balance on my bike seat with no hands it feels like I have to cock my hips to one side in order to stay balanced. Other than that I don't notice anything. I still lift heavy weights (well, heavy for me) and sprint and it doesn't bother me. I don't mean to encourage you to ignore your PT, just want to give you my story so you can decide if it applies to you.
3) I used an adjustable keyboard tray and would switch between sitting and standing at work. It was still uncomfortable but it helped a lot. I'd also take breaks to walk and lie down on the floor. This next part may sound kinda funny, but here goes: Sometimes I would have to sit for longer (meeting, car ride, etc.) and this would seem to mess me up for a while. So, I started trying to forget when it would happen. If I sat in a car for 4 hours to go on trip, once I got out, I'd try to forget it had happened and act like it was just a normal day. It seemed to help quite a bit.
Now that I don't have pain anymore I've gotten lazy and sit all day at work. I still exercise a lot though. It was a slow process to go back to doing things normally. I think it helped to think about the pain less over time. At one point I thought of it constantly. It never abruptly went away, I just thought about it less and less as it faded and one day it was gone and I didn't even realize it for a while because it had become such a small part of life.
Hope that helps, good luck!