Fastpass, tickets, and length of stay

Quick question:

We will be staying 13 nights our next trip with only 10-day tickets with WPFAM this time. When I go to book FPs, will it let me book for all 13 days? Obviously we won’t go to the parks everyday, but I don’t think we will know for sure which days will be off days until we are there.

No you can only book FPP for the number of ticketed days you have. So in your case 10 days of FPP over a 13 days period.