If you can look at how many points there are in the MMO and GPX tracks, that will give you a clue as to whether the GPX has been filtered. Since GPX is simply an XML file, you can read it with a text editor. You'll probably find the points are numbered, and you can read the number of the last point (if you have no other way of finding out how many points there are).
The fact that GPS Babel is unable to convert the MMO is one issue with proprietary, rather than 'open' file formats; there may be features in the file that are not documented, or known to outside parties (i.e. the author of GPS Babel).
BTW, why not download the GPX version of the route...?
LDWA Wales Coast Path
I'm not a member, so I can't help. And, since the page detailing how to join (from the link on the FAQ page) comes back 'Not Found' (due to extraneous quotes, it turns out), I probably won't be joining...