I agree! He wasn't defeated by Gothmog alone - it took several balrogs ambushing and restraining him for Gothmog to be able to land a killing blow. Justice for Fingon!!
Concerning Elros - it's not so much that "mortality took him", because he eventually voluntarily surrendered his life, as Eru had originally intended to happen for all men. Much like Aragorn later, he could have lived longer, but chose the appropriate moment to leave Arda and continue on his journey.