#111 (Z): If you want to reinforce a fight using Nydus worms you should:
-Rally your hatcheries to a nearby nydus entrance or exit
-Rally your Nydus exit to send reinforcements where you want
-Hotkey your Nydus exit and keep spamming "D" to unload units while you fight.
#112 (Z): A Nydus exit cannot be canceled once it starts building and only one can be built at a time but they can be queued so that the next exit begins building as soon as the old one finishes. If you want to build two exits at once you need two entrances.