Let’s break these questions down:
1 – I would put the Brand name you want on your products, and then file for IP Accelerator, because it will probably help to show that you are using the name. Though it probably won’t matter.
2 – If your first product is going to be part of the new brand, then just update the listing when you have the new branding on it. If you aren’t making it part of the new brand, just leave it as is.
3 – Then put the new product under the new brand and file for IP Accelerator right away (even if you haven’t started selling it yet).
4 – Then sell the new product. 🙂