It hasn't happened to me yet, but it's good to clear up your doubts before it happens.

I'm thinking of 2 Examples: 1: We are doing a Co-Op run, one of my friends loses connection, but comes back later and enters again, and we continue the run with him; and 2: He loses his connection and never comes back during the run, and we continue without him.

What happens if I submit a run in each of these 2 examples? Would they still be valid?

Which board would they be submitted to, if valid? The one with starting amount of Players, for example 4P, or the one with the finishing amount of Players, for example 3P if one of them left?

Was the specific board defined by the % of the time he participated in the run? If >50% of the run time would be the starting amount, if <50% would be the finishing amount?

In both scenarios you'd submit to the leaderboard the amount of people you started the run with