You have it Wilros, congrats
To put it briefly, the Quenya suffixes -ndil and -ndur apparently lived into Sindarin but went through the regular mutations of the language, hence nd > nn/n. Final vowels are also lost in Sindarin, thus ëarë > ëar. Therefore we have Earnil for Earendil and Earnur for Earendur.
Here's a blog post of mine explaining my user name:
Your turn sir