If you don't open the speed window, the FMC will gradually increase the speed to attempt to recover the path. If you open the speed window with speed intervention or use LVL CH it will keep that speed regardless of path. VNAV SPD will always comply with altitude constraints. Again, this is what I have experienced. Sometimes what the FCOM says doesn't seem to be incorrect i.e. when I had to discontinue an ILS approach and I deselected the frequency, the outcome was not what was the FCOM says it would be.
In your scenario, I would just report unable to comply with either restriction and ask ATC what if they prefer me on speed or on profile. And open the speed window if I have to descend faster.