Hello Pam - I love, love, love the
Hoop-Dee-Doo Musical Revue - you are in for a real treat! You have several options to get to Fort Wilderness from the Grand Floridian using Disney transportation. As you mention, you can either take the monorail or a boat to Magic Kingdom and then transfer to a boat headed to Fort Wilderness, or you can take the monorail over to the Contemporary and take the boat from their marina. Either way, I would start making my way over by 5:00 at the latest. You may need to wait for a boat so you have to factor that time in.
Since you are going in August, you have to be prepared for the pop-up storms that can occur around that time. If it looks like it's about to storm, I would opt to stay dry and take a taxi.
As far as seating, your table will be reserved prior to checking in for dinner. Although not official, I've read on the chat boards that tables are actually assigned when you make your reservation. I've sat in different locations in the venue and we haven't had a bad seat yet. The performers get everyone involved in the fun so it doesn't matter too much where you sit.
I'll bet you'll be singing Hoop-Dee-Doo after the show. Enjoy!