I can answer only the last question. Assuming that you can start both engines with one dead auxiliary pump (and you may be able to do so, more on this later), you could theoretically fly with the dead pump. In flight, the aux pump is only used for 3 things: keeping an engine running if you lose the mechanical fuel pump, restarting flow when you return to the main tank after draining dry the auxiliary tank, and restarting an engine that you previosly feathered. Obviously, with a dead pump, you can't do either of these. And I'm reasonably sure (but others may wish to jump in here and argue differently) that you would be breaking FAR regulations by taking off with a known defect in an item used to certify the airplane.
If the fuel system in a 336 is like the earlier version 337s, then with one dead and one working aux pump, it turns out that:
1. You CAN start both engines on the ground.
2. You CAN still do the three in-flight things which I said earlier you couldn't do.
But the system MUST be like the early 337s AND you need a thorough understanding of it, in order to use the working aux pump as a substitute (via cross-feeding) for the bad aux pump. More on this can be found in the "Fuel Supply Management" page at
www.SkymasterUS.com, including the procedure for starting both engines (look at the first scenario described under the heading of "Emergency Operations").
Ernie