Hello from a fellow Floridian!
I just did a the FLR(M) in person. The other week. I would say you don't need any of the miscellaneous stuff you listed. If you want to include the mortgage stuff though, it can't hurt but I think the rest of it is unnecessary. I've never even heard of an NHS medical card to be honest? Also the past p60's aren't necessary, just the last one :-)
Your passport photos I think need to be UK standard for the FLR (M).
Your application can be double sided but there is a bit in it where it says "Make sure this page is left blank on the opposite side if printing double sided" or something of the sort. I think it's the payment pages so just double check to do what it says if doing double sided.
You don't have to include your financial information.
You don't need to include a letter if you're going to add an itemised list, because basically all you can do in the cover letter is explain what is in the packet anyways, unless you want the space to explain anything but it looks like you're a pretty straight forward case :-)
If your husband includes a letter, it just explains his qualifying to sponsor you (husband, meets the financial requirements, and lists the financial documents he is providing).
I'm not sure about providing an envelope to send back as I did mine in person so hopefully someone else can help with this!
Also, I didn't see anything about pictures, but seeing as this is your second visa, I wonder if that is necessary or recommended still?
You're right about your question regarding the biometrics. You'll get a letter.
Finally, I believe that you cannot apply until 28 days before the expiry date of your visa so May 20th would be earliest. Someone else can confirm this but I believe this is the case.
Good luck!