Your users will be automatically updated to the latest version, which will be your webext. You, as the dev, cannot upload a non-webxt again to that listing. However your users can freely downgrade their version, they just have to make sure to turn of auto updates, otherwise it will keep updating them to the webext.
I’m not sure about this one, hopefully someone else can answer. I thought appending a or b does offer to go through to the beta channel.