The answer to the first question is: not very steep. I've gone over the grades before but have illustrated it in somewhat more detail below. It's perfectly feasible with a 1% grade; basically, you have about 1100 feet for the two lines to meet, and one can ascend while the other descends. This would put the grade of the track in the station at 14 feet—slightly lower than the current railroad—and about 10 feet lower than the adjacent streets in the BU Campus.
[Quick clarifying update on the above chart: dashed lines for the Grand Junction and Turnpike show areas where their alignments are outside of the constrained right of way area; for the Grand Junction, across the BU Bridge, and for the Turnpike, towards the toll plaza.]
The long and short of it is that the further west you move West Station, the lower it can sit. If it were moved to Cambridge Street, it could be at a grade of 0 feet, well below the current grade, although it would be much further from the core of the BU Campus. Wherever it is is a compromise: further east is higher (really anywhere west of Ashford Street would be a split-grade station with ramps or stairways between the two lines) and further west is lower. It would also be possible to have the Grand Junction in the center and the Worcester Line split on the outside; this might be preferable to easily enable cross-platform transfers. NB: Drawing not to scale: