I did try the G7 out of the D7 (directly, and indirectly via the Ab7) as a transition to C7 --> chorus in Fm (starting with the Fm chord). The G7 did transition nicely to a C chord, of course. But I felt the transition was so strong that it was hard to budge it off that C chord ... it didn't seem to want to transition further to my real endpoint, the chorus in the key of Fm (first chord of which is Fm). A C7 of course does do a better job of that transition, but it doesn't seem to follow from the G7 as well to my ear ...
So at this point, it looks like I'm stll favoring the D7 - Ab7 - C7 ... entry into the Fm key chorus, as "illegal" as it may be in terms of music theory. I am wondering though how much of that is due to having played it that way a thousand times ... if I were just working it out for the 1st time, would the G7 have sounded better to me? I'm not good enough to listen to the G7 with a fresh ear (and of course I can't remember at all if I did try it or not way back when I wrote the song, and maybe felt it wasn't quite what I was looking for)!!
I look forward to playing around with the "German 6th". I wasn't sure where the Db discussion came in though ... there's not actually a "key of F" in there anywhere ... I think I'm missing a few steps mentally there ...
Thanks again, everyone!
Alexis -Cubase7.5.20 64bit;i5-4570 3.2GHz,16GB RAM;W7SP1 64bit on Samsung SSD840 Pro256GB;Seagte 1TB SATA600 Audio;UR28M;Motif8;UAD2Solo;Jamstix 3.3;BCF2K;TC Helicon VoiceOne;RevoicePro2.5