The short answer to question #1 is yes. Others responded with their own experiences:

So far I have had some luck in modeling an off-line storage area with EXTRAN. My model network contained two such areas. One had three conduits entering the storage node and that worked ok. The other only had one and this presented me with some problems. The model would reach a certain point and then crash. I think this was because I had a terminal junction that was not specified as an outfall. My solution to this problem was to create a conduit near the top of the storage node an then connect it to an outfall. This seems to be working now.

I had one case with a large pond area that we wanted to use as a surge pond; The pond was designated as a storage node; this was connected by a pipe to a storm drain network node; the node received a surge from upstream in the network and passed part of it to the pond and part to the downstream pipe. After the storm drain flow dropped, the pond unloaded into the system. This scheme worked fine in EXTRAN, and it should work for you.