This provided at best incomplete protection. Delaying the inevitable as it were. Witness Spike through much of S4, dashing about under a blanket and venting smoke as he went. However, Buffy S6 establishes that vampires can operate with the Sun still relatively high in the sky, so long as the shadows are deep enough.
Yes, but see my post #5 above. The esteemed professor Vorkov (He was not mad, I tell you! He was a genius!) was prepared to carry out the very experiment you describe when the craven council of science dereed that the ban on sentient experimental subjects should be extended to vampires. And they say that Vorkov is mad? Fools! But fear not. Though they may have driven Vorkov to suicide, there are those who intend to complete his great works, the council be damned
There’s something about the radiant energy of the sun that the vampires clearly don’t like, but it is the wavelength or the amplitude? Or is it some sort of metaphor for the capital L Light (as in the powers of Good and Justice and so forth) and the capital S Son (as in the Son of God)?
This also raises the interesting question of whether a vampire would fry under something like the Icarus (the 21st Century Style Solar Laser from Die Another Day), even at night.