Thanks to you for starting the conversation.

Since they're widely cited, you have likely already read this research, but others who view this post may not have. These are among the broadest based papers I've seen to date on the effects of varying degrees of vegetarianism compare with other food habits in the US and UK.
http://jamanetwork.com/journals/jamainternalmedicine/fullarticle/1710093
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4073139/
(whole lot of additional reading to be found in the references of the second article)

Of course it's also worth considering that reduced mortality among vegans is due simply to the increased consumption of plants, which would be to argue that perhaps it is the case that the typical omnivore is malnourish due to poor diet in general, while the vegan may be taking better care to eat a healthy diet overall.

Fats are indeed a critical part of any vegan diet, and you are correct to note the impacts to brain function that could result from insufficient intake of the good fats. A vegetarian diet should always include portions of nuts, seeds, and other fatty plants like avocado or cacao. Luckily for us, our bodies make cholesterol from the primary nutrients: fat, protein, and carbohydrate. In fact, all of these are easy to consume as a vegan, although some care should be taken to be sure EFAs are complete. If there's one supplement a vegan should consider, it's an Omega-3 complex of some kind for the ALA.

There is more to veganism than just dietary or health benefits, and to many long-time vegans such as myself, those benefits may only be considered a pleasant side effect, or not even considered at all. Many of us in it for the long haul have chosen this lifestyle for environmental or very personal ethical reasons, like allowing grass-fed animals to stay that way until the end of their natural lives.

Cheers!