To answer first two questions:

Firstly, in case we can count open air museum as an example of adaptive reuse, open air museums are prime example of safeguarding vernacular built heritage.

Secondly, in case of my country (Czech republic) many people are owning/buying old buildings, especially timber houses and are repairing them while using traditional technology. These building are then used as a regular housing, as penzions, or as pubs. Many of these reconstruction can be label as a regular reuse or as a ordinary reconstruction, but there are projects, that can actualy be evaluated as adaptive reuse (I think I can find some concrete examples if you are interested, but I do not have them in my head).

There are several reasons, why people do this. They use the reconstruction/repair as a hobby. Many people and families also use it as a way to escape city live and how to find suitable living in a peaceful village environment. There are also projects made by local community with goal to save locally important building or to create focal community point.

For the last question:

From the concrete examples general methodology and "code of practice" can be made. On above mentioned projects, generally, people are working in cooperation with National Heritage Institute or with local heritage management office. Thanks to this, quality of work is secured. Also people are actively using this cooperation (and many NHI courses and workshops) to gain knowledge and experience, so they can effectively restore their building. On the other hand, NHI can use this practically gained knowledge to publish effective methodology, that can be used in "real world".

Also, e.g. if I recall correctly, government of Norway (or Norsk institutt for kulturminneforskning... I am actually not sure... it is a long time since I studied it) publish small documents (it was like 2-4 pages) about timber structure, windows etc. These documents describe construction of the above mentioned and also how to restore them, how to secure heat insulation without disrupting the original material etc. These are widely available and are passively helping to preserve original structures of historical buildings and its parts.

Best wisches,

David