Where can I find ban rules for Nintendo online?

Yea, when it comes to banning, Nintendo sucks! If you have literally ANYTHING that "may" be against their rules, then poof! No connection for you. I was actually looking into this myself. Usually, the ban is permanent (which really sucks), but you may try to appeal it by directly contacting them, though having a modded Nintendo would probably seal the deal from the get go. Anyway, here are the rules, I think rule number 10 applies to you. 

https://www.nintendo.com/terms-of-use/

Anyway, what you can do is spoof the address of your device, and delete the connection settings from your Emunand, and hopefully, the ban will be lifted (well, not really, you're spoofing your address). But here is hoping it works. There are tutorials online about this. This link may help you out:

https://www.youtube.com/watch?v=DzkBxo503ZY

You answered your own question. 

That's the risk you take when you mod a Nintendo console. You knew what you were getting into before you did it. You also knew that all is illegal in the eyes of The Red Behemoth and must be purged in the name of IP and copyright protection... FOR GREAT JUSTICE.

That's why I use an off-the-grid Raspberry Pi for my retro fix. The Red Behemoth cannot see my Raspberry Pi, and cannot smash it with the Ban Hammer. 

If you DO buy another "virgin" Switch, remember. You're not dealing with nice people who are understanding of your feelings, and will allow you to bend the rules slightly to get more value out of your device. You're dealing with a soulless monster that will protect it's IP from potential theft by ANY MEANS NECESSARY. 

Going online with a modded device is definitely one of the things on that list.

See https://www.nintendo.com/nintendoaccount/code-of-c... and it's point "C" on the list.