No one knows what 'spacetime' truely is. Space cannot be measured without time.
Humans can rarely grasp, if ever, something that exist without time.
"I don't know" is a perfectly valid answer, but one scientist are loathe to give. Not just them either.
1. As I understand it, space/time itself is neither matter nor energy. So no, space/time pre-exising the big bang requires neither matter nor energy outside of the singularity itself.
You nor I 'know' what spacetime is or isn't. As such, definitive answers such as your second sentence are at best a hypothesis, a wild ass guess at worse. We don't know what the singularity was comprised of, nor if it ever existed.
According to what you stated there, nothing existed. A null value. Yet the universe supposedly expanded into that null area. It's the chicken an egg argument. For there to be anywhere for the universe to expand into, something most likely had to be there as you cannot have an empty null space like that according to what current science tells us.
2. If you accept the theory that what is expanding is space/time itself, prior to the big bang, space/time was likely part of and/or no bigger than the singularity itself.
That's a circular argument that leads back to my answer for #1.
3. The speed (as we understand it) of light (or anything else) is something that can only exist inside of space time. Therefore even if the speed of light is an absolute limit on speed of movement inside space/time, assuming space/time is elastic and capable of expanding the speed of light would not be a limit on the rate of expansion of space/time. This is true for the simple reason that speed (distance/time) can not be used to measure the expansion of space/time itself.
Yet the speed of light figures heavily into many cosmological measurements. Especially that of sizing and dating the universe.
If it cannot be used as you say, then every estimate for age and distances within the universe is wrong. Another chicken and egg argument.
My preference is to say I don't know. I prefer to avoid 'assumptions' as that's the best angle from which to learn imo.