I am pretty much out of my league with questions as detailed as this, but from what I have been able to read over the years, In the late 1970's, physicists Chodos and Detweiler had a look at cosmological models based on 5-dimensional general relativity with an extra space dimension. As they followed the evolution of such a space-time, they found that the expansion was anisotropic and that one of the space dimensions collapsed while the others continued to grow large. Today, this process is considered to have preceded Inflation, but from what I can gather, it is not clear if it was Inflation which drove the change in the dimensionality of space-time by 'inflating' some dimensions and keeping others small, or whether the process of dimensional change is the trigger that started Inflation. I don't think physicists know enough about what quantum gravity ought to look like to be able to say with any confidence.
In string theory applied to cosmology, theoreticians have recently discovered that strings and quantum black holes represent two phases in the transformation of space-time at the Planck Scale. Like water freezing to ice, strings can 'freeze' into quantum black holes, so there is plenty of opportunity for odd things to happen that could serve as a driver for the Big Bang. We just don't know enough yet to know which processes are important, and which are not.