I wasn't quite sure of the answer to the park admission thing, so I looked it up in Carrie Hayward's book from "Passporters" (which I suggest buying if you're serious about moving forward with an Escape wedding).
I know that your guests don't have to have in-park admission to go to your wedding if it's held in-park, but they do need to be escorted to and from the ceremony because they don't have admission. So, after checking in Carrie's book, it seems to be the same deal with the dessert party. Your guests can be escorted in and out, however, if they haven't purchased in-park admission for the day, you'll just have to pay the viewing fee ($12.95 the last time I checked, but this may have gone up).
The book says that in some cases, you and your guests may meet a guide who will take you to your event--but in some cases, you'll need to rent Disney transportation for at least yourself if you plan on being in your wedding attire.
The book ALSO says (but this is something I think has changed) that only Disney photographers and videographers can attend the dessert parties. I do think this has changed, because I know I've seen recent overage from non-Disney photographers and videographers of dessert parties. I also haven't been told otherwise by my planner, and she knows I'm not using Disney. So, that's good--because I think I would flip out otherwise.
Carrie roams these boards often, so hopefully we'll see her pop in for more clarification. She knows EVERYTHING. That said, when in doubt, always ask your planner.
I understand the feeling of wanting to minimize the expense to your guests, but I had to decide for myself that if people weren't able to come because of the expense, then I would just go on ahead without them. Ultimately, it was my choice to have a Disney wedding--and I feel that's an imposition (financially) on everyone else to begin with (unless you plan on covering their travel, lodging, and food costs). With that choice comes the gravity of knowing that not everyone will be able to afford to come. The wedding is ultimately for myself and my fiance--everyone else is just a spectator. If having everyone from your family is more important to you, then I'd suggest the idea of having a smaller ceremony at home too (we're having a reception in our home state for everyone unable to come to the Disney wedding for this very reason). That, or just scrapping the full-on Disney wedding with the expectation of family being there and do just a Memories package for yourself and your fiance.
On the upside, just from my little tiny bit of research--it looks like you can get everyone to and from your ceremony and dessert party without asking them to buy park tickets. So, that's good.
Tiffiny