“Obvious” can be tricky to define. I’d say, though, that in most cases it should be safe to make the assumption. If one wanted to be pedantically correct, they could look up and check with the relevant legislation, but these declarations (see for example the one for Na Pho District, which should contain part of the missing section you mention) are made at the subdistrict (tambon) level, and are extremely tedious to navigate.
If a border looks like it follows a river, I’d say chances are it does, especially at the province level. An easier way to confirm would be to compare with available free maps / data sources, but those also seem to be lacking.
most administrative borders we have source from such an import. I would say accuracy of these is horrible.
I hope some day the local administration is convinced that OSM is useful for Thailand and they give us their boundary data to use.
Yup, I’ve seen borders which based on the pattern probably are supposed to follow a river in the NE, but a rather far off the actual river. So, probably should only be treated as approximations.