1. Can they see me start the race...where do they stand or go...and to do they have to leave as early as I am to do so?
The easiest answer to this is no...they let spectators go to the Epcot Parking lot and up to the Portapotties but to walk to the corrals, they only allow those participating in the race. So, they can send you off, but not see you start from your corral. The first viewpoint for spectators is a bit from the start - so really they don't see you start running, and personally, I would rather meet my friends and family at the finish line because to me, the race is all about finishing. 
2. Will the main st bakery be opened in MK that morning while they wait for me to arrive at cinderellas castle?
Remember that this race starts at 6 a.m. so nothing is open and I don't know if I saw anyone but volunteers by the Castle anyway. So, no breakfast for them, and if all goes well, you should really be done running by 10 a.m. I think realistically, everyone is well out of MK by the time the ropes drop otherwise they are probably swept.
3. Will they be able to see me cross the finish line?
Yes. This is what I recommend. Based on your running times, figure out what time you most likely may pass the finish line. Let your family sleep in and meet you 15 minutes +/- that time. There are stands that my family was easily able to wait on for me to cross. There is a texting update that you can sign up for and have people get updates as to what part of the course you are on, but I found it to be unreliable.
I know they plan on taking the monorail from epcot to MK and back to watch me. The monorails will be running at that time right
No. Spectators do have to stay in one spot from what I understand, so there is no traveling back and forth. The monorails start running close to the end of the race but I am not sure what the schedule is and I am pretty sure that they are only going from the resorts to the TTC. I believe that driving is really the only option for a spectator to cheer you on. Which isn't a problem because the parking at Epcot is pretty easy for the Princess. None of my family had a problem.
I'm going to respectfully disagree with some of this. I was a spectator cheering my friends on last year for the Princess Half, and I had a blast!
Some of it depends on how fast your runner is going....mine were at the very slowest pace allowed and were one of the last finishers, so it was easier for me to get around in time. But in general:
I got up at the crack of dawn with them and hung around with them in the EPCOT parking lot while waiting for the race to start. (It was COLD at 4 AM- but there was a DJ playing perky music so it wasn't horrible). At the last possible moment, they went right to get to the corrals (can't go there without a Bib) and I went left and walked a short way through a path in the woods to get to the road just past the start line. This was LINED with spectators. I will agree with the prior poster though that this spot wasn't really worth watching from. The runners were far away on the other side of the road, so I couldn't really see them and they couldn't see me, and the start line was far enough away from this point that I also couldn't see the fireworks/hear the anthem/etc.
Then I walked back on the path to the parking lot of EPCOT, and got on the monorail from Epcot to the TTC - this was sometime between 5 and 7 AM, I don't remember for sure, but I can tell you it was neat walking past the front gates of EPCOT and seeing it completely empty except for the one lone security guard! Matsuriza (the drummers from Japan pavillion) were playing at the TTC with about 50 other people joining in - it was pretty awesome to watch while I waited for my runners!
After they went by, I walked on the path past the Polynesian to the road on the other side by Shades of green (sorry I don't have a map in front of me to know names, but I know it's after they ran through Magic Kingdom) and watched them from there. (If I remember correctly, it was about 3 miles later on their running route, but only about a 10-15 minute walk for me)
Then I walked back to the TTC, hopped on the monorail back to EPCOT, walked to the parking lot, and watched them finish. I will tell you that this was the most stressful part of the morning - everyone that was in the Magic Kingdom area watching racers was trying to get back to EPCOT, but so was everyone trying to make park opening at EPCOT, so the line was INSANE and I was worried about making it in time to see my runners. However, when I was actually on the monorail, it was neat looking down and watching all the runners on the road below! I thought briefly about taking one of the buses from the Poly to EPCOT instead of the monorail, but because of road closures I heard they were struggling to get there as well.
My runners really appreciated me being on the route at the TTC and ESPECIALLY the road after the Magic Kingdom. They said that was the most boring stretch of the race for them and it helped to have someone cheering for them there. The problem is, if your runner is fast, you may not be able to make it back to the parking lot at EPCOT in time to see them finish if you do that.
Disney was REALLY hyping watching runners at the start, and then the Magic Kingdom, and then the finish. However, the runners I talked to said they didn't NEED extra enthusiasm while they were in the Magic Kingdom - how could they when they're running through the castle??
I wasn't actually in the Magic Kingdom so I dont know for sure if anything was opened. I can tell you you could get coffee at the TTC, I got Kona Coffee at the Poly (and Captain Cooks, the food court at the Poly, is open 24/7 so you can get breakfast there) and while I don't think Main Street Bakery would be open, I do think there are probably some street carts or something open to purchase coffee, etc at Magic Kingdom. It looked like there were a lot of spectators there, and I can't imagine Disney missing an opportunity to make money...erm... feed you!
I loved the experience of cheering my friends on, and met lots of cool people along the way!
My parents and I are driving down on Friday, and well go to EPCOT that evening after the EXPO for dinner (no World Showcase). Saturday will be a leisurely morning at AK and then well head over to MK for dinner at LTT. I thought it would be good to show my parents where we will be coming into the MK during the race, and where well leave the castle area. Im going to try to be in bed between 9pm and 10pm. I hope I can sleep! Sunday RACE DAY!, and afterwards Im just going to go until I drop. It is EMH at MK, so I thought wed go there that evening. Mon and Tues will be spent going to the parks, and well drive home Wed.
The places where I saw the most spectators were the TTC, Entrance to Epcot and the Finish Line. Which were really the best places - I agree that the last thing you need in MK is extra motivation. If only the race was through MK constantly, everyone would get a PR from all that pixie dust and a good picture running through the Castle! 
