If something always existed then that something always had properties and drive. If nothing always existed then that nothing is really something.
I am NOT a credentialed physicist, so take everything I am about to say with a big mountain of salt, but this is my take on it:
'Nothing' is a very misleading word that I really wish physicists and cosmologists would stop using. What they really seem to be describing is not some ontological absence of anything at all, but a mathematical 'nothing' or zero. Despite what your teacher may have told you, zero is NOT nothing. In fact, zero is everything! Zero is the sum of infinity. If you add all the positive integers to all the negative ones what do you get? They cancel out to zero!
As for your other questions? I'm clueless. Some physicists have speculated that the impetus and substance came from another universe, but that is just an ad hoc hypothesis that pushes the question back ad infinitum.
To be honest, I'm one of those crackpots whose very skeptical of Big Bang cosmology. If the universe is expanding, I haven't seen it. All we have for it is the word of folks who get paid to come up with theories, form conjectures, and push numbers around all day. There are several rival theories, like Plasma Cosmology and Null Physics, which do their best to explain Hubble's Law in the context of either an eternal, beginingless cosmos or a much older one. When so many intelligent people disagree on this subject, it tells you how little evidence we truly have to go on.