With all due respect, but I think this question should be asked on the steam support forums and not on itch.io, since they are both game stores, I feel that the question is a bit inappropriate.
If the moderators and/or the administrator don't mind your question and because I don't want to be rude and not answer, I'll risk telling you about my experience on steam.
That 46% seems too optimistic to me, and a value closer to 30% would be more realistic. The main error I see in your image is that you have 0% projection for discounted sales.
Most game sales are at launch (typically 10% to 15% off) or at special events like Christmas (typically 25% to 50% off). Unless you plan to never make a discount.
1) No. One thing is the percentage that you receive for each copy actually sold and another very different thing, it is to calculate an estimate based on the value of the full price in the USA vs the total estimate of sales in the world.
2)As REFERENCE values, they don't look bad.
However, whether or not they are real for your game will depend on many factors, for example, if your game has a demo, you will have a better return rate than if you don't have a demo.
If your game has Russian language, you will have more sales in Russia than if your game doesn't have that language and the regional price percentage will be higher (It also depends if you agree to use regional price). etc
In the absence of more data and details and as a starting point, these values do not look bad, but you must be clear that an average value is one thing and dispersion is another.
3) Yes.I) I have a 6%.
In the sales reports that I have access to, steam gives me a single field with chargeback+refunds and does not inform me of the customer's reasons for requesting a refund.
I don't have data to answer further, but I do know that there are many players who "buy" a game to try it out, they use it as a form of demo.
II and III)
These questions are more related to statistics, what you understood is correct. But you are trying to calculate an estimate of future profit. You really don't know how many copies you're going to sell in the US or how many copies you're going to sell in Russia.
Inferring these values depends on many factors and can be very complicated, a simple way is to use numbers of similar games.
However, you must be clear that these numbers are not a law, but an estimate and a very simple one.