You're the customer, but you're a business customer, not a consumer customer. B2C vs B2B is different, and the contracts involved are different, and it's different way of thinking. The Internet I have at my house prohibits reselling because I'm buying it as a consumer. If I'm buying a business Comcast account, there's an expectation by Comcast that I'm going to be reselling the Internet access (like if I own a coffee shop or something). Thus, imo it's not common, but it's also not prohibited. (But I am not a lawyer and this is not legal advice.)

As for having two integrations, what's your opportunity cost? You'll want a backup integration, but imo that's in the same category as having a backup cloud to run on in case AWS goes down. Which, you do, but the time spent working on that is time spent not working on the product.

Yes, this is a very common practice especially for larger sites. There are even services like Rebilly and Spreedly that simplify this process.

Very common. Mistakes happen, resolution can be slow.

You might be able to justify a single payment gateway integration if you're MVP in a simple consumer retail business.

If being unable to take orders for two weeks would be a big problem, then make sure you have at least two gateways, and keep them all warm.