Will Apple disable your account for buying IAP abroad?
On the "can you spend a Gift Card on In-App Purchase" part, here's what Apple says:  


  You can use store credit for purchases that you make in any of the stores or to pay for in-app purchases until it's gone.


As long as you keep using Gift Cards and don't run out of credit you should have no problem.

If you have or get a local credit card, and want to use it, you will have to either create another Apple ID to manage both stores, or switch to your local store and lose US store purchases, as you need your card and store country to match.

As Apple puts it:


  For example, only German credit cards can be used to buy content from the German iTunes Store and App Store.


This articles 1, 2 take on the problems and advantages of having more than one Apple ID for purchases and describe what you do in extension.