I mean, professional cyclists do the same, as do migratory birds. Only the front has to take the resistance, the back has to spend less energy. The slow, tired ones just drop off.

This is a guess but the fluid dynamics likely changes? As the size of the group increases the Reynolds number probably increases making inertia a significant factor.

Individual human runners in air is maybe a bad analogy think more like honey where every stroke pushes you back as much as it pushes you forward.

This article might interest you https://doi.org/10.1529%2Fbiophysj.107.118257

Well... if its mucus that's causing the resistance then the sperm need more power to overcome the resistance. So if a group clump together in a way that focuses the forces of their movement on a small section of mucus then they'll work like a group snowplow.