Veganism 'the healthiest diet?'

[Edwardo Piet]

So, TL;DR version:

[...]my question is twofold...

1. is mainstream science really in favor of vegans? Or is the reason why vegans regularly fail to link to non-partisan sources so often--when arguing for how it's supposedly the healtheist diet, healthier than even pesecatarianism--because they value their moralizing above their intellectual honesty?

and

2. Does all the mainstream hard medical scientific evidence really say that chicken has zero health benefits (I've read that women who eat chicken are less likely to get breast cancer) and--more importantly--is it really really the case that fish like salmon is bad for oneself and veganism is healthier than all forms of vegetarianism? (I sincerely doubt it, the way Wikipedia says that vegans have lower mortality rates than meat eaters but vegetarians have lower mortality rates than vegans seems extremely commonsensical to me... anyone got any better sources?)