I hate to ruin your chance to be dismissive...
I'm not being dismissive. I'm saying you're wrong.
You insist on assuming that the length of time between the rise of life on Earth and the rise of multicellular life is indicative of the odds of it arising--and you apparently (please correct me if I am wrong) base this on the assumption that multicellularity arose from internal forces. I'm arguing that we can't know that; some EXternal force may have pushed them. This would throw your calculations out the window. IF something external caused a push towards multicelluarlity, THEN it logically follows that we should only count the time between that event and the rise of multicelluarlity when we run our calculations.
Paleontology works by bad analogies in many cases. In this case, we don't have the original fossils to work from. Ideally, we should look at something that's gone from unicellular to multicellular under conditions that allow for observation--which means, in the last 200 years. Such examples don't exist. So we are left with tow ways to examine this question ("Let's just assume" isn't a valid test). We can look at things that appear to be going from unicellular to multicellular, such as slime mold--or we can look at bryozoans, which exhibit an astonishing level of differentiation, such that the colonies of the species in question can be thought of as meta-organisms, with each individual organism having as much autonomy as the cells in a more typical organism do. Since we're working with bad analogies, we want to work with both; they have different errors, so will tend to cancel each other's errors out.
If you have EVIDENCE that nothing external happened to push organisms toward multicellularity (no, that's not asking you to prove a negative, and I really shouldn't have to explain why), let's hear it. Otherwise, it remains an untested assumption, and I see no value in assuming one untested assumption over another.
I will pass on a new thread. I can tell from this exchange how it will end, and have lost my taste for futile debates on this forum.