Thanks for the feature request. Adding custom descriptions isn't something that's in the current plan (this is the first I've heard of the feature request) but it's something I'll consider. I can definitely see how it would be useful.
The ability to choose which products get submitted by creating a custom collection is one of the most popular feature requests for the app. That feature isn't on the roadmap to get the product out of beta, but it'll be one of the top priority features after the app is officially launched.
As for editing the metafields for a product, you'll find a section called "metafields" on your product pages in the Shopify admin. You can edit these to set the MPN, Age group, Gender and Google Product Type for your products. To set UPC / EAN / JAN / ISBN for a product, you can edit the Barcode on a product variant.