September in the Tuolumne Meadows area can be hot or cold, depending on the year. You are not going to be at really high elevation. Day temps are usually still warm but there is a chance of light frost at night. It has snowed and rained in mid-Sept in the past. Best bet is to check the weather report just before you go out (NWS internet sites are good and their 5-day forecasts are pretty good). I pack for expected weather but always bring an extra layer in my car, and if conditions change when I am at the trailhead, I add the extra layer. And because it could be quite hot, zip-off legs on your hiking pants would be very useful.

The trials are likely to be dusty so light weight short gaiters are nice just to keep all the dirt out of your shoes. Also, on your route and the low mileage, trail runners would suffice. In my opinion, nothing beats Smartwool socks.

I think a 25-30 degree bag is OK. Around 2 to 2.5 pounds is about the right weight. "600 vs 650 vs 700 vs 750 vs 800" down is the quality of down - not temperature rating. You can find 800-down bags rated from 40-degreesF to -40degrees F. The lower the down quality, the more weight of down fill is required for the same temperature rating. I would aim for 750 quality down. I personally would avoid 600 quality bags. There are lots of sales on now - get on the internet and look.

There is no such thing as an all-season bag. Most of us have separate summer gear and winter gear. If you are interested in doing more Spring or late Fall or higher altitude backpacking, then a 10-20 degree bag would be best (these usually weigh 2.5-3 pounds). True winter backpacking is not something I would suggest beginners do. Get some experience first.