I think the person who finds the mark (and their group if they have one) should always get full credit. They did the work to look for it, they got lucky and were the first ones to find it, they deserve it. No one else deserves full credit. They may be lucky enough to get there in time, and that's great, but they weren't the ones to find it and thus aren't entitled to it.

To get full credit, the person / group who's found the mob have to be the ones to make the pull, especially if they're not a full party. This means they will have to pull as soon as they find the mark / their group gets there, before the group of 200 people arrives and claims it as theirs (even though they weren't the first ones there).

Resetting mobs is claim stealing and reward stealing, it's intentional griefing and should always be reported.