Third, we have MoS Shiro Emiya. Kiritsugu 2.0...yes please. AFAIK, MoS is the unofficial third EPIC ENDING of HF.
Hell no. After the way he betrayed Sakura, all that asshole deserves is an agonisingly painful death.
Also, MoS wouldn't be "epic". Assuming Shirou does somehow find a way to win, it would be seriously anti-climatic, because if he gets into any actually epic battles he would be seriously outclassed and would die without a doubt, particularly given that Archer is still around and has absolutely nothing preventing him killing Shirou any more. He'd just hide in the shadows and then snipe the enemy masters or whatever. It would be extremely dull and depressing.
And, honestly, if you don't like "dark", I don't see how you can possibly
like MoS. MoS is the darkest possible path Shirou can follow, and ends badly for him and everyone he knows.
Regarding Heaven's Feel...I hated it, TBH. It's too dark, and Sakura's backstory (which ironically enough is her biggest appeal for me) is just too damn cruel.
Well, Sakura's backstory is definitely cruel, but I don't think that makes the route as a whole bad. I think Nasu is very good at giving characters horrible backstories....
I also know the Matou have somewhat of a sinister reputation, which makes me wonder why Kiritsugu never bothered to investigate and preemptively strike them down before the Fourth War.
I don't think the Matou are particularly well-known, honestly. Tokiomi certainly didn't seem aware of Zouken's true nature (although he must surely have had some idea), and he's far closer than Kiritsugu is. Plus, Zouken is hardly a threat outside of Fuyuki, so there are bigger things for Kiritsugu to deal with.
Out of the three routes, the only one I really liked what UBW (idealistic Shiro in Fate and broken Shiro in HF canon both repulse me for some reason) because there Shiro learns how not to follow his ideals, but simply to use them to temper his own life.
Honestly, I dislike UBW because it's played as happy whilst Sakura is left to be tortured to death by Zouken whilst her sister and the guy she loves run off to London together.
Which is actually how things are in reality; we don't follow our ideals blindly, nor do we simply abandon them - we use them as a guide for life.
Honestly, I think HF Shirou fits that just as well as it does UBW Shirou. He doesn't simply "abandon" his ideals, he just accepts that, to be human, you can't really consider all lives equally. He still tries to protect everyone else, he just refuses to murder Sakura in cold blood to do so.