What do we know about Shirou's biological parents/grandparents/etc.? Do we even know anything at all?
This is relevant to e.g. understanding why Shirou can perform magecraft despite presumably having had non-mage parents. (Also, it's just kind of weird that he seems to never say anything about them. He was certainly old enough to remember them when they died in the fire at the end of the fourth grail war.)
The identity of Shirou's biological family was never revealed in the game, the anime or the manga of Fate/Zero and Fate/Stay Night. The only thing you know about them is that they died because of the end of the 4th Holy Grail War.
As for the reason why Shirou can perform magecraft, It is caused by the sheath of Excalibur : Avalon, which have been implanted in Shirou's body by Kiritsugu. Even though it is not clearly explained.
To answer your first question,
I would like to say that there has been no information released regarding it in the anime/manga/visual novel/games/wikia site. However, there is this piece of information on the wikia site.
Ten years before the start of Fate/stay night, Shirou was an ordinary boy living with his parents in Shinto. The great fire caused by the contents of the Holy Grail spilling out at the end of the Fourth Holy Grail War killed both his parents and left him mortally wounded within the blaze.
Therefore, we do know that his biological parents died during the Fourth Holy Grail War.
To answer your second question,
Again, the wikia site states that
Shirou persistently asked Kiritsugu to teach him Magecraft. Kiritsugu eventually complied even though he disapproved of Shirou's desire to learn his craft.
That is why Shirou Emiya can perform magecraft.
To answer your third question,
All I can say that there is no definitive answer to this question owing to the fact that there is no reference to this in any of the aforementioned media.
As revealed during Epic of Remnant within Fate/Grand Order (An alternate timeline), Pseudo-Servant Sengo Muramasa uses the body of Shirou Emiya as they are compatible. Muramasa later states that there is a possibility of Shirou being his descendant.
Ok, really? One, Shirou doesn't remember anything before the fire. The fire literally distorted him, making his mind inhuman enough to house a Reality Marble, and because of his survivors guilt as that happened and his awe at the person that saved him like a hero, heroism was practically imbedded in his soul. Distortion is akin to becoming Eldritch to a small extent. It's not normal and he lost his memories in that blaze. Two, he can perform magecraft because he was born with Circuits. However, the reason Avalon is the reason he can use magecraft is because Kiritsugu half-assed his teaching and he did it wrong, turning his nerves into makeshift, temporary magic circuits everytime he used magecraft instead of using his actual circuits. This usually crippled people but avalon healed that damage and allowed him to practise the almost suicidal method of magecraft. Rin awakened his actual circuits during the war.
The reason why he had circuits, the background of his family, name or true age, no one really knows. And why would Shirou talk about something he doesn't remember and holds no attachment too because he doesn't? Besides his one-tracked mind towards self-sacrifice that stops him ever caring about himself that is.
