Glad to see you got your system going again.
The reason why there are few instructions for 'downgrading' is that it is an inherently risky process, and likely to break the system.
It is relatively easy to create new stuff that is backward compatible
with the old system, and can just be upgraded in place.
On the other hand, with downgrading, you are replacing components with older versions that might not be forward compatible
with the rest of the system (because one cannot predict the future).
Anyway, yes, it is possible to force package managers such as apt to downgrade. One can use pinning
to force it to install older versions of packages (have a look at the second section in this howto
However, I would not recommend this unless you are prepared for the possibility of ending up with an unbootable system.
If you have an option of simply reinstalling the older system (not forgetting to backup your data first), it is faster and involves a lot less frustration.