Downgrading isn’t possible, if you’re on tests-passed you can’t rebuild on a lower version (beta/stable).
You would need to stay on the current tests-passed release until Stable or Beta catches up.
Subfolder installs are considered an advanced topic and we really can’t support them here because of all the problems which arise. There are pages and pages of topics detailing the types of problems found when installing against a subfolder. In 2019 there’s really no good reason to use a subfolder install, all of the SEO claims have been debunked, and the added complexity really adds nothing of value.
Even on CDCK hosting, subfolder installs are only available to enterprise customers, with an additional fee.
I would suggest reverting to a subdomain, or opening a topic on Marketplace to engage with a consultant.