1) Shepard isn't a time traveler. A solution to any problem is a combination of time, energy, and resources. Solving one problem means another problem doesn't get solved. Indeed, that's the entire basis for certain morally grey scenarios, since the player can't achieve all possible positive outcomes.
Add on top of that, many tragedies are orchestrated long before anyone can feasibly predict its happening. Go tell any one individual person to prevent World War II, during that time period. Or to cure cancer, once discovering a loved one was just diagnosed with it. You'd be laughed at.
WWII is the sort of thing I've already discussed, about the larger tragedies requiring going further back but their individual impacts don't. Cancer boils down to simply bad luck at some point (although obviously influenced by other things).
2) If you are capable of making all tragedies simultaneously preventable, then I'd say you should be made Emperor of the World. Unfortunately, most individuals recognize that not all tragedy is preventable, at least not given any technology or resources we currently have available.
Because we never have all the information available. If you could metagame life then you probably could.
3) No, I completely understand your point, it's just very weak. You think that right and wrong solutions are superior, because player agency makes tragedy more tragic and happy endings more happy. Unfortunately, you're ignoring the fact that in scenarios where the player is choosing simply between tragedies, he is still demonstrating player agency (see Kaidan or Ashley). The player can still be made to feel all those exact same emotions, only without the metagame disappointment that Bioware tends to write cop-outs into many of their morally ambiguous scenarios, depriving them of any intellectual consideration. It's like pointing to the trolleys thought experiment, toss the person an easy-mode solution, and then claim it still has the same intellectual integrity.
Now that is a weak argument.
If you've got a morally ambiguous situation that truly is ambiguous then it's no choice. If you think that's what's needed for intellectual consideration then there's no hope at all. The intellectual consideration comes from trying to work out which is the best option, even if none of them are perfect. That's why for me the ME2 Geth Heretics decision was the most interesting in the entire game - it combined consideration of which would have the biggest impact (good or bad) on the rest of the galaxy with which would be considered the worst intrusion on a form of intelligence completely different to ours (does the nature of the geth mean that they're effectively reprogramming themselves fundamentally all the time anyway, so doing so might not be quite as reprehensible as it would be to us?)
All this talk about easy cop-out solutions entirely misses the point I've made a few times where I say that they need to be very hard to achieve - just like life. Are you not just shying away from wanting to have to take responsibility for your failures?
I'd definitely argue that the player doesn't feel quite the same in Virmire because they know there's nothing they could've done to prevent it. Are you really trying to suggest that there wouldn't be a bigger impact on the player if they were left with a dead squadmate they were responsible for, wondering what they could've done to avoid that?
4) Sorry, but you're doing a bad job of stating whatever it is you think you're stating. If for some reason, you think good and bad outcomes make your tragedies more tragic, it's not place to tell you what to enjoy. But your point seems to go beyond that. Apparently, all those players who do enjoy morally grey territory and moral philosophy, which by the way is an entire subject in itself, are doing it wrong, merely because Reorte has a different emotional reaction to a video game.
If all you can do is to start to sneer at someone who doesn't share your opinion ("are doing it wrong, merely because Reorte has a different emotional reaction") then there's little point in discussing anything further with you.
As I've stated several times, you have an extremely limited understanding of the potential for the gaming medium, whose strengths are simultaneously to make a player feel powerful or powerless, in addition to other functions. What you're suggesting is more akin to saying that films shouldn't implement music, because it's a different medium, which most would say is ridiculous.
You've stated it numerous times whilst being incredibly guilty of it yourself. A game is a fictional medium where a player can take responsibility for their actions and the consequences of them. That is it's huge strength and one that you are for some reason blinding yourself to.