Hello and welcome.
No, the upgrade assistant is used on a previsou version, like 7.17.X to see if you can safely upgrade to version 8.X without any issue, once in 8.X you will have an upgrade assistant when version 9.X is launched.
No, this is not possible, you cannot downgrade Elasticsearch.
If you are having issues after the upgrade you will need to troubleshoot this issue to try to solve it.
I'd recommend that you open a topic with the error/issue you are having and share your configuration and logs.