With the caveat I'm summarising from what PBS Space Time and Dr Becky* say:

• Big Bang: we can only see back to surface of last scattering, i.e. the CMB, extrapolating backwards goes "???" at much the same point as it did a few decades back because we still have not unified quantum mechanics and general relativity

• CMB should only have isotope distribution of Big Bang nucleosynthesis, that hasn't changed in the last decades, dunno if that's what you meant by "various non-uniform gases were scattered around"?

• Variations in density of CMB do exist, key phrase is "Baryon acoustic oscillations", while they're very small magnitude they're also massive in distance scale, so they're how galactic clusters formed (that scale rather than stars directly): https://en.wikipedia.org/wiki/Baryon_acoustic_oscillations

https://www.youtube.com/watch?v=PPpUxoeooZk

https://www.youtube.com/watch?v=LRUTnoveZs8

• Re: "Perhaps the expansion of space wasn't uniform either": I heard about specifically "Timescape Cosmology", but a quick search says that's part of a broader category of inhomogeneous cosmologies: https://en.wikipedia.org/wiki/Inhomogeneous_cosmology#Timesc...

https://www.youtube.com/watch?v=SXg6YVcdOcA

https://www.youtube.com/watch?v=JlNVZz5D6WE

• Re: "and when space expands or contracts, energy is generated": no, general relativity does not in general conserve energy, and it is related to the curvature of spacetime. Simple example is that the photons in the CMB have much less energy to us than they did to the atoms they were emitted from**: https://www.youtube.com/watch?v=04ERSb06dOg

* I assuming I'm correctly judging the level and attention to detail they're providing, given the detail they put in and references to specific research publications. My degree is Software Engineering.

** There's also a Veritasium video about this, but to me Veritasium feels like a BBC 2 evening popular science show, so I'm not as confident about recommending it.