1) It is in the range of 1% and you've missed the rest of my sentence which is within the first 3 years of their release. Shadow of Mordor and Tomb Raider came out fairly late in their life cycle.
I've said that out of the big publishers only 2K seems to want to support Linux currently XCOM2 was the only "AAA" game that came out with day 1 support of Linux performance issues aside (Pretty much all the big games on Linux on Steam have considerably higher system requirements and do have performance issues, Shadow of Mordor is near unplayable).
2) 2000: a lot of them are indie games. Whether those are borderline games or not is left to your opinion, but I personally have several hundred games on Linux that are actual games and not "mobile-games ported to Linux" kind of games.
Again which games, I have 3714 (I have a Steam account since HL2 like most others, and on this summer sale alone I've spent nearly 400$ already on games) games in my Steam Library some of them do have Linux support (Valve and a few 2K games) but for the most part none of the games I or my friends play come with Linux support
3) Wine has improved but you again missing my point completely 5 years ago I could play most games on Wine today due to lack of DX11 support I cannot play any new games on it, yes DX11 support will eventually come but atm the library of new games I can play on Wine is smaller than what I had around 2006/7.
4) Vulkan is now but there isn't much to any support for it as of yet, it's too early to tell what's going to come of it, but I doubt it will get a large scale adoption it's just the matter of things, OpenGL tanked, Mantle tanked, like it or not DX is king currently.
5) Well I don't know about you but I remember when I could play WoW, Warcraft 3, and Call of Duty etc. on Linux using Wine on day 1 pretty much these days it's not the case so for me there are actually less titles I would want to play on Linux, but hey different strokes.