Hi @BabetteY
Vivaldi is made up of many things. First there are the browsers for desktop and Android. Then there are the services - Sync, Webmail, Forum, Blogs and Themes - for which you need a Vivaldi account.
For the most part, you can use the browser without a Vivaldi account. And you can use Vivaldi services (excluding Sync) in any browser. For example, you don't need to use the Vivaldi browser to post on your blog on Vivaldi.net. Speaking of which, your blog looks alright. So, when you log in on Vivaldi.net and go to your blog, you should be able to post on it without issues. In case you can't, please share any error messages you see.
Having so many different services, unfortunately, means that they all have their own settings and need to be managed from their respective pages. @DoctorG already shared where, for example, Forum settings can be found.
Also, uninstalling the browser won't delete your Vivaldi account and/or unsubscribe you from newsletters/notification emails. Deleting the account will unsubscribe you from any messages that were linked to your account.
Regarding the first issue you mentioned about losing tabs. We're very sorry about that. It shouldn't happen, if you have enabled opening the browser with the last session. In case this happens, though, a good place to check is Recently Closed Tabs (trash can button on the far side of the Tab Bar).