Can I use INT./EXT. for my action scenes?
A master scene in an action movie may have many quick twists and turns. Example: A car chase. You have a car, a driver, and a road. But if your headings bounce from INT to EXT to CAR, to Road a dozen times in one page, the spec script looks discombobulated. Can you use EXT. / INT. to cover both the external movement of the car and the internal movement of the driver?
Quick answer? Absolutely. Your objective, especially in a spec script, is to create a document that reads as much like a movie as possible. Breaking what is essentially a single scene into a dozen or more INTERIORS and EXTERIORS would have exactly the opposite effect. As you've suggested, you're much better off using a single INT. / EXT. to introduce the scene, then short action lines to track movement from beat to beat. A scene written in this way will be easier to write, easier to read, and will feel much more like a movie.
FYI, John August covers this topic (and many others) in his extremely helpful website, Screenwriting.io.