I would like to credit back a portion of my buyer's down payment on a house, say a few days after closing.

As Chris mentioned it works the same, however, the tax credits and incentives may be different for a primary home vs a rental.

In your example if value is not sufficient, they will hold you each accountable and if your four partners disappear, the bank can still hold you personally responsible for the entire deficit.

What's the best business bank account and the best business credit card?

They will also be able to work with you to help you get better credit or optimize finance to afford more.

One strategy I like to do is build in seller credits into my offers so the loan covers the closing costs, e.g. if you wanted to offer $500K for a property, offer $510K with $10K towards closing costs and that $10K is supported by the loan and can be used on the costs I mentioned above.

If your going to be mixing all of these accounts and then going for a self directed roth IRA later, this will apply to you. 

If you are just getting out of high school, I don't know that a large DSCR loan seems like a good idea or possible, but maybe easier if you haven't built a ton of credit yet.

You'll need to find a small local bank or credit union that might lend to you.2) Will your grandfather cosign for you to compensate for your lack of a job?

Thanks We typically like to use credit unions.