1) DER-mot
2) I like it, though it's not one I'd use myself.
3) Nope, I don't know any Dermots.
4) Not sure.
5) Eh, it's so-so. It fits in quite well with Emerson and Lachlan, not so well with Niko. Your boys' names are sort of a mixed bag (not criticizing...I like that about them). It would be hard to find a name that fits well with all three. In your shoes, I might look for a name that is a little closer to Niko, stylistically, but ultimately I wouldn't give up a name I loved just because it makes Niko more of an outlier.