Would appreciate any comments regarding this premise & any pro/cons in visiting the island in early Sept. vs. Dec.
I'm not concerned about differences in cost primarily the weather.

Sept. runs the risk of hurricanes so consider trip insurance. Dec starts prime season so you should find most restaurants open. Nice weather. winds pick up in March.

Great upscale restaurant down road from Mt Vernon on the right: Sol e Luna. further down same side Le Taitu more casual excellent local food dishes. Could possibly walk to either. Using a car drive toward Pinel Ferry dock (down Mt Vernon Rd take right) for more good places to eat.

Anyone know if the road to Mt Vernon has been repaired? It was in terrible shape when we were on the island in March.

The only con for us is really a pro also. If you stay at a mt Vernon property, you're on your own. We were long time sonesta maho people but now have fallen in love with mt. Vernon.
We spend Christmas and New Years there and the weather is perfect. Last trip we didn't even rent a car--took a cab to Marigot one day but that's it. It's a nice walk to club orient where you can rent chairs and enjoy (imho) the best of orient beach.
There's a pizza shop at mt Vernon that delivers to your room.

The road to mt. Vernon has been in "terrible" shape for a long time. I think it's a perpetually inevitable condition.

You know Mont Vernon is no longer a "Resort", right? It's all private residences with privately owned apts to rent. Everyone I know that rents there likes it. The road has not been repaired and is full of potholes. Sept is low season, so some restaurants are closed, beaches are quiet and it's Hurricaine season. Dec is high season and more crowded.

When we've walked down there, it seems as though there's more seaweed, and the water is rougher........????

Yes, there has been more seaweed at that end of the beach a lot of the time in the past year. The waves and the undertow are a little worse at that end of the beach.