Steve, I know teh area quite well too.
Back to the collision : if the press info given so far is correct , the PA32 was EDLS to EDFC , so tracking 120-130 about , and the DR400 was EDRK-EDFB, so tracking about 090 . the good old converging , not moving target, add the (very low) sunshine . the PA32, about 50 Kts faster than the DR 400, here you go.
Pure speculation of course.
Class E and G in this area around FRA TMA maybe fine , but seen the traffic patterns around it , include the business jets cancelling IFR to pursue VFR at 250 Kts ,and in the volume of traffic in the WE especially , not contacting Langen INFO for as transit is crazy. (but not mandatory, we agree )