You need the latest Platinum version/update installed, and then it works. You copy the m2ts files on your hard drive, you use the "match media" icon on the project properties and then you edit. There is no special workflow. Yes, you can burn a DVD.
I don't understand what you mean by intraframe. Are you sure what you are seeing is not just some interlacing? Because most consumer AVCHD is interlaced you see, with horizontal lines.
When I read about the frame structure of how AVCHD records in a long-GOP format with an I-frame, followed by B-frame and/or a P-frame, I don't understand how all frames of an event look like an I-frame (with full picture information).
Do you really need to know? I mean, it works and it looks as it's supposed to. What more do you need to know if you are not a developer who writes encoders?