Regarding the 240% Echidna, it's 220%. There is a cap on passive i.e 50%. Therefore 60% Fire ele up of Echidna + 50% Dark Ele up counted as Fire ele up for SSR Amon = 110%.
Secondly, that "if" for mixed element is a big if which at the moment we don't even know if it's possible or functional.
Besides Bear mentioned that she does just fine for a mono team. No reason to divulge to mixed unless 2 SSR Diabolos'.
Also grids would also suffer unless you have null weapons so no point agonizing over it. She is pretty useful for most content on a mono Fire team even if her damage is not as high as some of the famous/infamous fire SSRs like Uriel and Svarog.
The important part of her kit is the strong buffs and team BG gain regardless of the nuke's damage (Unless you wanted another busted hime for Fire).