That most consumers will come to Apple with their laptop for a service and Genius will replace the battery. Will genius replace the battery for their AirPods?

Yes (though in practice, they're probably just swapping the unit and sending the original back to be refurbished). https://support.apple.com/airpods/repair/service

When they “replaced the battery” on my dads iPad Pro they just sent him a new one. I don’t know if that holds for MacBooks as well, it might depend on the model: the 12” MacBook’s layered battery might be pretty hard to replace.

No, not really.

Practically, an iPhone battery is replaceable in you average mall tech shop in a mid sized town. In the 5/6 era, it was a 30 minute job if you’re being slow and careful. (I’ve got 3 screen replacements and 2 battery swaps under my belt from that era).

My understanding of the AirPods is that the complexity level is more akin to component level fixes for a main phone board. I’d expect that there are people who can do it, but they’re rare, and they’ve got specialized equipment, and it’s probably not worth it for an earphone battery.

There are repair shops for 'most consumers'. They do great work, but even they can't fix an Airpod.