Hi,
I think the Olympics and the Paralympics will work fairly similarly as far as entry to the park goes.
if you have a ticket to an event on the park, you can walk around the park as long as you want...the park is BIG and walking from one end to the other takes about 40 mins, most of it is flat/wheelchair friendly, although there are a few inclines and there are additional walkways on different levels (but you don't have to take them!).
If you don't have a day pass, you can only get into the venue you have a ticket for, and each venue is completely enclosed with only one entrance (the stadium has a couple of different entrances/bridges - but again impossible to get into it without a ticket). I am not sure how the day pass "availability" thing will work if the Paralympics are sold out, but you may be right about them allocating a certain number of seats for people with day passes (I only had tickets for events, so not 100% sure about this) but I did read that there is no multiple entry/day pass for the aquatic centre/stadium/velodrome.
There is loads to see - gardens/big screens/etc etc, so although it would be nice to go to a couple of different sports, there is enough to entertain even the most hardened soul in the actual park!
As far as getting there 90mins before your event - we didn't really find that necessary, the main thing is to give yourself time to get to the park without having to rush, but its fine to arrive at the venue 10 or 15 minutes before the start of your session.
And Minnie - the jubilee line goes to Stratford so you wouldn't need to take the dlr - unless you wanted to?