Jason,
Indeed the Dirac string answers all of your queries if understood in its appropriate context and background information.
1. The invariance of lightspeed is a function of the timespace, which is algorithmic, that is it is abstract mathematical without any spatial or duration based prerequisites.
Time in timespace requires an ordering principle, say like event A must occur before event B can occur.
A particular algorithm in timespace computes 10 fundamwental constants of nature, including h, c^2 and k. Those elementary constants are integer based and universally constant as number strings.
c is then an upper bound for velocity and also a lower bound for tachyon hyperspeed of say the de Broglie wave matter hypervelocity of the inflaton.
2. The QBBS expansion from the singularity mathematical point changing into a pointcircle I have addressed previously. Much detail, including the nature of the graviton can be found in the linked reference.
Now should you consider your graviton expansion as the actual expansion of the universal lightpath and understand that the graviton is one Goldstone gauge agent within the unified field of quantum relativity, then you could utilize your graviton expansion model in a mode of closer approximation to that of the multidimensional cosmologies.
The Dirac monopole as a pointparticle of creation so can also be related to the graviton as the quantum gravitational agent in a necessary unification of 5 Goldstone bosons in energy-momentum conservation.
1.Electromagnetic gauge-source-sink photon
2. Electroweak gauge-sink-source antiphoton suppressed by the weakon mass inducers including the emergent Higgs boson
3.Strongweak gauge gluon in 8 permutation eigenstates
4.Graviton for Gravitation
5.RestMassPhoton as the precursor of the Higgs Boson and as the dark matter gauge, also defining 'physicalized consciousness' as a dark energy defined in the quantum angular radial-independent acceleration acting on any spacetime volumar; again answering your query regarding spacetime origins.