Still doesn't explain why it has to be Shepard's DNA or essence or whatever, at least for synthesis. It's just a contrivance so that the writers can sacrifice Shepard to make the ending feel more dramatic.
Very true.
But really, the entire synthesis option is utter moronic nonsense. A beam of energy that passes through all life in the galaxy, analyses it while passing through, designs a way to make that life synthetic, creates out of nothing the resources necessary to transform that life and then transforms that life on a cellular level without making any mistakes, all within a fraction of a second. It's ridiculous.
There is absolutely no explanation as to what physical effects it uses to accomplish this wonder. Certainly not any we know of in the ME universe. In fact, using the same kind of technology it would be trivial to make a device that creates a mystical wave of green energy that passes through a planet, turns all members of a certain species (e.g. humans) into goo, creates out of nothing a storage box within which that goo is placed and a communication device for these storage boxes which signals their location. Would've certainly made the reaper's job a lot easier. One wonders why they didn't create such a device.
It's a punch in the face for anyone who gives a **** about ME being sci-fi.