Ok, we've just returned today and I don't have all of the information for you that I would like to have. DH and I went through only three of the houses and all of the scarezones. As we walked through, we tried to pay close attention to any problems you may have.
As was mentioned before, sometimes the fog from the machines is quite thick and you may have a difficult time getting your bearings. I would second the suggestion of some kind of light on the chair to lessen the likelihood of someone falling into you. There were quite a few people there who had a great deal to drink and the number of intoxicated patrons increased as the night wore on. I think that, coupled with decreased visibility, may cause an accident if the chair were not easily visible on the street.
I know the light wouldn't be allowed in the houses, but I also don't think it would be necessary. If the crowd size the first two nights are any indication, you will have someone directly in front of you and someone directly in back of you, sort of like a large conga line going through each house.
The first house we went into was called "Run" and it seemed to be the least accessible. There was a section with open doors to the right and left of you on the hallway which you had to turn to get through. I honestly don't know how a chair would make it down that hallway. They may close the doors slightly, or divert you, but otherwise, I don't see it happening. The other two houses had tight corners, but seemed to otherwise be fine.
I asked an employee in a motorized chair if she had been to the staff preview, but she had not. However, she indicated she had also been told all of the houses were accessible.
We did not go through all of the houses because at times, the lines were over an hour long and we are just not that patient. If seeing all of the houses is a priority, I strongly suggest that you get an express pass.
I know this was not very helpful, but I hope you have a great time.