@Raya I put your answers in bold
I know I can't get hotel reservations until June 28, however, at that point do I need to buy tickets immediately to reserve park days?
You cannot reserve park days without ticket media, given the complications of everything, I would book a package if possible (reservation + tickets)
Can I reserve park days before I buy tickets - to ensure that I'm not buying 7 days of tickets but can only get into the parks for 4 days?
You can see what park days are available before you book a reservation or buy tickets
Is this system even going to be in place next May?
That's the 64 million dollar question
What about park hoppers - they were what I expected to get - is there any way to buy them just in case they're back?
According to the information posted, they are selling park hoppers with 2021 tickets
If we have hotel reservations for one hotel, then switch to another do we lose our park reservations?
We don't think so...I assume it's like FP
We have a person who might be joining us but won't know until after March, if I get park reservations for our party of 5 then want to add them is that possible?
Probably 
.
You can always remove people from a reservation in advance of the 30 day package deadline. Err on the side of they're coming?