I concede the point on the slats. Found the same in web searches. Interestingly, I also found that the first model of the DC-9, the DC-9-10 did not have any leading edge devices at all.

However, while there does not seem to be much if any wind tunnel data confirming the performance of high-lift devices on models, there isn't any disproving either. My own experience with model-size aircraft using LE slots, LE notch (NACA droop), flaps and slotted flaps indicates to me that they work as well as the full scale implementation. Just my non-quantitative experience, FWIW.

As far as a trade study vs complexity, etc. THe slats would be an interesting exercise, but if designed into a wing from scratch could be built with little or no weight penalty, and with simple enough systems to be reliable. Quite a few have done so with models of the A-4 skyhawk.

As for TE flaps, having a single slotted flap implementation is no more challenging or heavy than a plain flap. Basically, it's just a different axis of rotation and some tiny but crucial details that make it work.