To answer your question Cloudy, all pre-boards are given time before general boarding to board at their leisure. This also occurs for families with children, wheelchair passengers, etc.
As for UM's - we don't carry them. As of last week. That may change...who knows?!
The only seat allocation (if you can call it that) is when people with guide dogs, etc. have to be allocated a seat in the first row. But generally the Ground Staff will bring them on first to ensure they get the seat they need to sit at.