I don't carve my pumpkins until no closer than three days before Halloween, depending on the weather.
I DID hear or read once about someone liberally spraying Lysol inside and out to keep them from molding and rotting-- but I WOULD NOT recommend that.
Why not get the fake carvable pumpkins from the craft store if you want to keep them longer?
I haven't ventured into decorative carvings. I enjoy the simple Jack-o-lanterns and REAL tea light candles so the scorching pumpkins make the porch smell like pumpkin pie
-- keeping an eye on the Jack-o-lanterns and the door for the trick or treaters is just part of the Halloween tradition.