hmmm astronomically higher? let's see if you run the same leve twice and a mob gives 100 sp per kill you get 100 sp for the first run and 100 for the second run for 200 total. if you run it linked with another you get 100 + 60, but have no second run. by linking you actually lose sp overall, but it makes running leves more time efficient than running 64 individual leves with an 8 person party.
not if you fail all your leves. even if you run the "big 6" and run 2 others that is gaining guild marks at 1/4 of the time as others so, yes, you are giving them up. this was about the failing leves and what do you give up. if you have guild marks on a leve and fail it then you do not get them.