Underground longevity totally depends on your soil chemistry, moisture, abrasiveness, and composition (some minerals are more reactive than others).
Hardness of the copper is the same the dufference is Type M wall thickness pipe is thinner than type L. In otherwords a given pipe size, for example 1/2" pipe the outside diameter is the same for both type M and L but the inside diameter is less for type L copper since the wall thickness is thicker. Type L copper is stronger pipe due to thicker wall thickness.
Yes, I use it all the time
Yes ... copper pipe is permanently marked. Here is a link telling you about that ... https://www.copper.org/applications/plumbing/cth/standard-tubes/cth_1stand_id.html
Copper pipe is fine for air , it works best if you use a mapp gas torch . I generally use 3/4" galvanized for air lines .
Possibly, but why does it matter? Pipe is pipe. There are regulations, so China, Mexico or US pipe is all the same.
Yes. This pipe can be soldered
Standard length carried in stores is 10' They do carry unions to extend the length beyond 10'
Yes, this pipe is pure copper
No, it will tarnish over time.