Ps4 – If I login to a PS4 from a different region to the PSN account can I use a payment method from the the PS4s region to make purchases on the account

Yes, it will work. You can buy PSN cards online, such as on Amazon, but make sure you purchase a US PSN card. Once you've purchased it, a code will automatically be sent to your email. If you have other accounts and want to buy PS+, all other accounts will also have PS+. The same goes for DLCs and downloaded games. However, this can happen only if the US account is the primary account.

In addition, make sure all accounts are set up before buying PS+. I made this stupid mistake of making my US account, purchasing PS+ and then creating a new user, so that new user didn't get PS+ because I created it after buying PS+ on the primary account.

For primary accounts on a console there can only be one per account. When you make a new PSN account on his console this will make that PS4 be your primary console (I believe it does this for brand new accounts). You will not be able to do cloud saves without paying for playstation plus, but your trophies will be saved to the cloud by default since it is related to your PSN account. To answer your questions directly:

1. This will not damage your friends PSN account.

2. This will not do anything wrong to his account or PS4, though when you get your own PS4 you will need to deactivate your account as primary on your friend's PS4 (if by chance it is primary) or you will not have access to any purchases you may have made or will make.