Transferable DLC between accounts, and regions?

I own a physical release of Splatoon 2 on my Nintendo switch. The cartridge is European but I am playing the game on an American account. I wish to buy the octo expansion. My main concern is this:

1)If I buy on the USA eShop, will I be able to play the dlc on the EU cartridge?

2)If I buy on the EU eShop, will I be able to play the dlc on my US account?

According to Nintendo support, you need to buy a version that is compatible with your version of the game. You can check this by pressing + on the Splatoon 2 home icon and selecting "support information". (I assume this means you need an EU version, but I'd double check to make sure)

If you're able to play the game on your US account now, then you should be able to play Octo Expansion with it as well. The games are supposed to be region free, but this gets complicated with DLC, as in this case.