No evolutionary clock models used, instead a numerical model was used.
1) The rate of change not being constant is not significant as long as its variation is normally distributed about some mean. I expect that changes in environment are randomly distributed.
2) This is too small scale to have any impact on the trajectory of the numerical growth in number of base pairs.
We don't need to guess, life isn't special. The growth of complexity under favorable conditions is observable and occurs all over the place at the same rate, at scale.
Life went from minutes per generation to a year per generation!
Mutation rates of RNA-based life was likely 1,000x higher than later DNA based life!
How are these “not relevant”?
It’s like estimating the velocity of a ball from a replay that has slow mo, time-lapse, and the game being played changes!
Intelligent life on earth is DNA/RNA based, not one or the other. If you take the mean rate your interpolation from now back the origin of such life with that particular mean rate of numerical growth in base pairs holds with error margins.
https://en.wikipedia.org/wiki/RNA_world
Your assumptions are faulty.