If I am adding words both from the queue and from a custom list, will the words be added from both lists simultaneously, or will the queue first be emptied, and thereafter the custom list?

It mixes and matches. Sometimes it will add from the list, sometimes from the queue.

Aha. Good to know, thanks!

On this topic, I would like the queue to be pausable if at all possible.

We'll be working on turning the queue into more of a list, with controls and browsing and everything, in the next month or two along with some vocabulary system overhauls we're doing. Then it shall please you.