Welcome, Astra, and I'm smiling because I couldn't wait to ask all those questions when I first joined the forum!!
Get ready to become a little confused, though - in fact, you'll probably end up VERY confused!
First . . . there are SO MANY "legitimate" interpretations of Astro Boy, his story, the names of the other characters, and different endings, and powers and how he works, and what he runs on . . .
The short answer is -- there is no, one, correct answer to your questions.
I saw his spikes kind of "wobbling" in the movie, too, so it seemed like maybe they were rubbery.
If you watch the anime from the 1980's there are a couple episodes where he crashes and actually DENTS his "hair" so it has to be replaced! (Sea Serpent Isle is one of them)
Back in the 1960's anime and also in the original manga you see Astro sometimes drawn with his boots, and other times not!
In both the 1960's anime and in the 2003 anime you'll see close-up how the foot of his red boots "collapses" and turns into a jet . . . BUT in the 1980's anime, "Cathy" from the robot circus gave him the boots (they were hers when she was a girl) - and throughout the series you see him take them off, put objects into them . . . but he also flies with them on (which, of course, DOESN'T make sense!:rolleyes
As for his "skin" - he appears to be "shiny" (especially his head) to one degree or another, in every incarnation except for the 2009 movie. I agree with you, that any child with metallic, shiny skin and "hair" would NOT look like a real human to me! Yet they portray him as fooling people all the time. I guess folks aren't too observant in Astro's world!!
For whatever reason, Dr. Tezuka (and now, his son and Tezuka Productions) have continually made changes to Astro's appearance, age, powers, and story over the past 60 years, and some of these things ARE in conflict with each other, but there really isn't anything we can do about it . . . Tezuka was completely comfortable with retooling Astro as he saw fit (and when he actually lost the legal right to create more anime using Astro, he plagiarized himself by making an Astro Boy copy named Jetter Mars . . . and that's ANOTHER whole story!!
But confusing or not . . . it's fun to speculate, and personally, I believe they would give Astro realistic hair, and some kind of soft-to-the-touch covering so he wouldn't be shiny and metal-looking.