When we did the post-party, CMs with light sticks helped clear the path so we could very easily get from the garden to the terrace for the dessert portion of the event.
Yes, pre-party guests can hang out in the garden while Main St. clears out - we always do this if we do a pre-party. The crowds heading out after the fireworks are really, really bad.
The garden viewing area does feel a bit crowded, IMHO, but certainly not as crowded as the hub will feel. If your husband has difficulty with crowds, I would probably opt for the post-party. You may want to try to arrive early and explain your husband's needs to the CM hosts to see if they can get you a table near the railing.
I've done the seated party twice and, while the view is a bit on an angle, I was never disappointed. It is a nice respite from the crowds.
Just a word of caution, there is zero guarantee Tink will fly on any given night. It really depends on weather conditions and Tink has the final say as to whether she is flying or not. I would pick the party that is going to work best for your family and consider Tink a bonus.