Great party title, Romp in the Swamp. You're off to a good start.
I've bought scene setters and some other brand of scenery before and they're okay. Most of my stuff is in the roll format which still has folds. If lighting is low you don't notice the folds and seams as much. Could you link to what in particular you were thinking of using? BTW I've used most of this stuff outside attached to fences and outdoor walls, so not sure how well it works with the recommended sticky tack indoors. You might experiment with using a hair dryer on a warm setting to see if you can release some of the folds and wrinkles before applying the tape. BTW
Shindigz is a good place to look for scenery and party items.
Here's some ideas that may or may not be what you are looking for:
Thinking of keeping the budget low, you could see how tea-stained or green-dyed cheese cloth works for you if you want to add some Spanish Moss-type hanging elements from your ceiling, lamp shades, etc. Maybe add some Great Stuff or glued on cotton balls material for some texture. Before putting it on something test it to make sure the color doesn't transfer.
If your party scene is an abandoned haunted Southern Mansion you could cover all the furniture with white sheets and use candles (real or battery powered) for lighting. If you use some lamps inside maybe connect them to an F/X lighting box so that the lamps can be dimmed and flicker.
Cob webbing is pretty inexpensive--don't forget doorways, hallways, around mirrors.
I always associate fine old New Orleans mansions with beveled glass windows and items like silver candlabras and cut crystal (search out resale shops like Goodwill for bargains on this kind of stuff)--just an air of elegance.
For food in the mansion area you could always serve tea sandwiches to help stretch your food budget.
You can cheaply make some family portraits by printing out photographs, age them with a tea, and throw them in frames from places like Dollar Tree or resale shops.
Look online for some inexpensive rubber alligators. Maybe one in your kitchen sink, in the refrigerator or in a bathtub would be a fun element of surprise. Of course one coming up from a toilet would be sure to get some reaction from people--you'll definitely need another bathroom for them to use of course. And I'd probably put a sign on the raised toilet lid saying "out of order" just to be safe from drunk and oblivious party goers who really have to go.
Back in 2005 I saw the movie THE SKELETON KEY with Kate Hudson, Gena Rowlands, John Hurt and Peter Sarsgaard and would
highly recommend watching it for ideas for your party. It's set outside of New Orleans in an old run-down mansion near the swamp and hoodoo plays a big part in the movie. It's a great thriller. I picked up a copy of it at ROSS DRESS FOR LESS or BIG LOTS a number of months ago so know it's still out there being sold. You can always rent it too. Here's a link to it over at IMDB:
The Skeleton Key (2005) I jumped a few times in my seat during this one

so hope you enjoy it.
BTW Yubney, the face you are making is terrific. I assume it's paper mache? That's a great, inexpensive material to learn to work with and it's amazing the kinds of things I've seen people on this forum make with it.