I don't know the legal ins-and-outs of multi-use paths, but my own take on them when they cross a street is that bikes must stop or slow down.
If an auto driver stops to allow you to cross, I figure they're just being nice.
I then cross carefully, never assuming the other vehicles are going to be nice and stop too.
Here's a column by Ray Thomas trying to figure out MUPs. He seems to say that bicycles on an MUP should slow down to walking speed when they cross a street, similar to the rules for bikes on sidewalks.