To decide on which bags to use, I started by looking at what was in the bag. then I had to drink what wa in the bag before I mixed xtol and filled it up. Of course I chose 5L of wine so the bag would comfortably store the full 5L of developer. The funny thing is someone here posted an amazon link with unused box and bag for $25, when a 5L box and bag with wine in it costs $18.99 locally...
You may want to consider reinforcing the boxes around where the spigot protrudes - packing tape can work.
I have a friend who built a jig to make it easier to extract the tap. In some cases, with wine bags, the taps work fine even if they are not fully inserted into the collar that they go into. If that applies to the ones you have, they are much easier to remove.
Depends on what kind you got (AstraPouch makes a ton). The type I buy has a regular threaded cap on one side, I rinse them with distilled water and reuse.