Shades,
There is little chance of sitting at a cocktail party as they seem to be a mingle affair, so keep you heel low unless you are comfortable in high ones. You can always wear a cute wrap to help with the shoulders, and your dress doesn't have to be low cut and there is no reason to expose the chest. A nice halter neck LBD might be the way to go; and depending on your age it doesn't have to be short, it is totally acceptable to have to the ankle LBD. In my opinion, some one my age looks out of place in a LBD a twenty something would wear (sadly).
The one I went to I got a professional to do my make up, which was great.
You can also take peoples looks away from the torso by having cute ear rings, guys and girls like the look of danglingly ear rings.
Cocktail parties are an excuse for girls to show off so be aware and I'm sure you are.
BTW the one

that I've been to, which of course makes me an expert, the girls drank very little (maybe one or two cocktails?), the guys drank a lot. The girls were very into comparing fashion and the guys were very into comparing girls.

But it was a lot of fun and I would go to others.
Hugs
Cindy