Does surface scan have any meaning here? Say, if someone (real first) did a basic system scan with his advanced scanner only, will the next person who would do surface scan be credited with 'discovered by' tag?
"Discovered by" is on a per-body basis, not a per-system basis. You must use a detailed surface scanner on an individual celestial body, and be the first to sell the data related to that body, in order to receive credit. You do not receive any credit for just using a discovery scanner to receive the "undiscovered" markers, nor do you get any credit for scans obtained without a detailed surface scanner.