1. yes, it doest affect the monsters spawn level, Each individual trap door you enter increases the monster Level by 3. and it stacks, so if you enter several trapdoors in a row. you could be potentially killing monsters a dozen or more levels higher then you.
the EXP, is capped though, at the level of the monster the driver is.. so the only thing you get is a little increased drop rate ( untested) and Higher level drops ( tested and duly confirmed)
Question 2, im not sure, i believe that the monsters stay XX levels above you, but, again, untested (at least on my part)
hope i was helpful

good to see you again
