That Goldilocks zone is (drumroll please) about .6 of an AU - which is about 90 million km across.
Actually, there's one other very well evidenced explanation for the diversity of life on earth, and a whole heap of really poorly evidenced ones.
Funny - concentrations of CO2 literally an order of magnitude higher than we have not have existed in the past 40 million years, and life didn't have any problems dealing with it. Similarly, CO2 concentrations less that half of what it is now have occurred in the past 2 million years, and ligfe had no problems there.
No seasons would mean that life was different (probably), but no axial tilt and no seasons really doesn't matter that much to most lifeforms. Certainly it wouldn't affect most sea life, all archea and bacteria, all lithtropes and most other micro-organsiams, which make up the vast preponderance of life on the planet.
We MIGHT not be around, but then again, humans have only been around for a very, very, very short time in the history of life.
If gravity were different, no universe and ergo not life. Or possibly not. There are some models that suggest a the gravitational constant could swing upwards/downwards by 4% and things would still be much as they are.
You might want to go back and check the tape on that one. To the best of my knowledge, they physical constants have been pretty much "constant" since the very early universe. Any changes in them have been in the order of parts per million or parts per billion.
Gonna need to see your maths on that one chief. Care to outline how you made your probability calculations?
You really don't know much about early universe cosmology do you? I don't either (really), but I do know enough to shake my head at just how awful your description of the Big Bang is.
Wiki has a nice write up on the timeline of the early universe, which provides a nice springboard to dive into the longer/deeper descriptions of the formation of our universe.