My manuals give a nominal value of 771Kg/hour for CWT scavenge pump flow. However, this may vary.
In theory, fuel should not get into the venting system because of float valves fitted to the vents. However, improper seating of valves, fuel sloshing arount, etc, may allow fuel to get into the venting plumbing.
There are flame arresters and pressure relief valves for added safety.
I just checked our manuals and the HST pumps have the same part number/s as the Override Jettison pumps. If you look at the Fuel synoptic, the pump symbols should be the same size (at least on the
real aircraft).
Can stab tank pumps provide fuel to the crossfeed manifold as other boost pumps?
If you mean directly, then only by engineer intervention on the ground.
Rgds.
NSEU