That would be more expensive than shipping all shipments by airplane. They would have to make the switch way more expensive

So does that mean that they'll be making a loss on each Switch sold?

That's assuming they didn't spend the time working to increase production.

So how were they able to ship more than 2M in March?

They're probably making a little bit of loss on these, but I guess it's worth it.

Did they ever do this with the Wii?

"another Wii U"

Not every Switch was sent by plane. Basic reading comprehension is required. All it means is that some units were sold with additional cost overhead. We don't know how many or what percentage of total systems they represent, in order to be able to calculate the actual loss or loss of profits. 

That takes time. Even if they did while they flew in units, the new extra produced units would be stuck on ship, at least until May. But I assume upping the production is more of a process that takes at least one or two months, so results of increased production wouldn't be available before late in summer.

Readthe OP, they shipped parts of the April shipment per plane, so it arrived in March.