1: light when considering it's speed is a form of energy and so has no mass. when it interacts with other things it is considered a particle but i don't believe it is considered to have mass in either case.
2: Why are we speaking of chemical propulsion systems? we probably would reach such a speed only with a nuclear reactor and so the energy released
by the fuel would increase as the mass increases. A small percent of the mass of the nuclear fuel is transfered directly into energy: Energy=mass *
the speed of light ^2 : therefor as the mass of the nuclear fuels increases so does the amount of energy released, and so because the mass of the
entire... thing increases, the thrust in proportion to mass remains the same.
in other words if your using a nuclear propulsion system, you'll keep going, until you reach the speed of light, then well i can't figure that out
to any degree what so ever, hey I'm only 16 give a guy a break.
3: the reason we speculate on these things is because we want to learn. For me at least, it is also because i want to see the wonder Gods creation,
and the complexity of it all. I believe that God set in place the laws and constants of the universe and then set it in motion, but who am i to assign
a truth value to it, no one can guess at the mind of God and say that he/she is correct, only say that they believe that it is true but not say that
it is true.