Larry, you make it sound like it's simple. It's not. OPC is trying to change a lot of other architecture that has specific business and security reasons for being the way it is.

Until a way is found to make OPC accommodate the way the AIM checkout is done, you'll have to temporarily live with the "popup"/review stage.
Or use something other than AIM, such as Square, if it works in your country.

Lat9 is working hard to accommodate the complexities of exactly what you're asking for. The "true OPC experience" you mentioned IS INDEED her goal. But please tighten your patience hat until the technical glitches are sorted out.