Correct, after 13 weeks, as long as you keep the file and have enough uptime, the expected income will turn into actual income.
You spend money as you upload. So if you only upload 10 GB, you'll only spend a bit of money. And if you upload 10 TB, you'll spend more money. But you'll spend the same price per GB (There's a baseline fee that you also have to pay, which makes 10 GB look a lot more expensive per-GB than 10 TB, because at 10 GB 99% of your expenses will be the flat-fee, but at 10 TB only like 0.1% of your expenses will be the flat fee)
SIAs monetary foundation is it's own coin, SIA coin, everything is calculated in that currency.
if the price of the dollar goes up and you use dollar to buy SC, then you are right. But remenber that the price is calculated automatically, so if the net work's host are happy with the same price in dollar as before the rise they could lover their prices in SC keeping it stabile in USD. I think it is still too early to see how this correllates given the very small demand for storage and the in all respect little competitiion there is between the small number of hosts at the moment.
If you quit the allowanse, there is a part of the fund you will loose, but since you then have used no storage, you have no spending on that bit, so you will get most of it back once the contracts expire.