I've done OpenVPN to NordVPN (I've even played around with 4 tunnels and load-balancing on the 4 tunnels)
But haven't been able to configure IKEv2 towards NordVPN. I read the guides you mentionned, but from what I read, MSCHAP can be configured for an IKEv2 server on pfSense, not an IKEv2 client on pfSense. The guide on IKEv2 that you linked to is written for a IKEv2 server on pfSense, and remote clients like IOS or Android.
Here's what I did:
download root certificate from NordVPN
convert to PEM format
import as a CA in System->Certificate
Go to VPN->IPSec and setup a sit to site tunnel.
However, in the authentication box, either I see "Shared PSK" or "RSA"
I have tried both settings, selecting the Root NordVPN cert for the remote in the "RSA" mode, or using my NordVPN password as the pre-shared-key when in "PSK" more
When I go to the status page, and click "connect", it goes back to the "disconnected" state almost instantly. When I check the logs, I keep getting an authentication failed reply from the NordVPN server.
I might be missing something, though :o