I don't remember anything about mtb:scale=6, so it might be newer than then I have read these ratings. But looks like mtb:scale=6 isn't 100% impossible, so it would be good to use some other rating for impossible or forbidden places. mtb:scale=-1 have be in use pretty widely, so it's in use even it isn't a standard. An in that I checked that place were you said I used that rating. There I have added mtb:scale=-1 because that path goes to a private area, so yes it would be technically possible to ride but it's allowed.

No, that isn't a typo. Number and symbol "-" means that is a little bit easier than number (1-, 2-, ...). Number and "+" is a little bit difficult. Zero is no difficulties. But -1 is negative value meaning mountain biking is impossible

A one possible typo is that I have added the old name but I have forgot to change a current name. Mustalahdentie is the real name in town of Noormarkku, but after Pori took possession of Noormarkku, they changed names of many roads.

I loaded objects, but I forgot to make a changes to x number of objects, before I uploaded :(