Dear MrsReither,
I don't know what type of brainstem tumor you have. But the decision to avoid "pushing" (valsalva maneuver) by avoiding normal delievery, seems to be very reasonable for me.
Hope you are doing well. No regarding your pregnancy, but did take Keppra for seizures caused by Temporal lobe tumor pre and post surgery. Side effects were a bit bizarre, including depression/temper tantrums that could be more challenging with your pregnancy. Hope you are doing well.