Not quite - Supersnoop has the timing right:
From the first day the first family takes their first picture, they have 45 days to download that day's pictures before they expire (disappear from
MDE).
From the day that first download is performed, you have 30 more days in which you can ADD pictures. Each of those must also be downloaded within 45 days from the date taken before they expire.
Use caution when trying to wait until exactly day 45 because the site is slow and you could encounter difficulties, making it a stressful experience. Although I have heard good reports about people being granted a free extension in those circumstances, I wouldn't want to oush it if I didn't have to.
The account login to view MM photos is the same one used to log in to MDE for reservations, FP, etc.
@Mckymousefn - as far as I know, once someone is in your a Friends and Family list, they do have the ability to see and change your Fastpasses and reservations. I don't think there's a way to let them share pictures but not other plans. When my family shares, I just make sure to tell them very clearly NOT to mess with anyone else's stuff!