Regarding your first question, if I am reading this correctly your smart campaign would look like this:
(1 or 2)
Member of Smart List A
Member of Smart List B
If that is the case then you are correct, leads one only receive one email regardless if they exist in one or both of the nested smart lists.
For your second question, I believe your options are either to have the smart lists nested in your smart campaign like above, or manually recreate the filters. If you do leaved them as stand-alones, you will not be able to delete Smart List A or B as long as they are in use by a smart campaign.
Thank you Emily. Yes that is my smart campaign.
Thanks for confirming my assumption as well, guess there is no shortcut to merging lists.