I am no expert on making adjustments, so perhaps another user might give advice on that.
I do wonder if the fact that your issue with with single notes (and not chords) may suggest that you "looking for problems" when you hit single notes. An issue with your expectations rather than anything really wrong. It is something to consider, anyway.
You could try some of the FXPs for the D4 and K2 which might help you.
The fact that you have a different experience with different frequency ranges could indicate an issue with headphones. Pianoteq is a precision audio application and some headphones distort different frequency ranges. I can say that changing from one headset to another I notice a definite difference myself. I don't think you need expensive headphones, but it's not generally easy to find what's right for your own preferences. It could be as simple as the cable connections.
It's interesting you mention that you find the D4 more delicate for Bach, as Bach never used an instrument and did not compose for an instrument like a D4. Keyboard's in Bach's time were quite different and the Graf and Schantz are much closer to what he'd have composed for (and on). These instruments do encourage a slightly different approach to playing these pieces. There are things that are natural on a forte piano than don't work at all on modern grand piano the same way. Some of the way we expect to hear Bach now is influenced by mainly hearing it played on grand pianos, but if you listen to recordings on historic instruments you'll start to realize that they were written for that kind of device and work better in some cases on them. YMMV but keep exploring. And try the harpsichords as well.
Mixing means that you can (CPU allowing) run more than one Pianoteq application at the same time. Both will register the same keys and you can, therefore, play two instruments at once. You can do this in a more sophisticated way using a DAW application and using Pianoteq as a VST instrument (google some of this) but it amounts to playing more than one instrument at the same time. A VST is a virtual instrument wrapped in a particular kind of software interface than allows applications that support VSts to use them. A DAW is a Digital Audio Workstation and they are software applications for sequencing and manipulating audio, especially MIDI controlled audio systems. Pianoteq comes with a standalone application but also comes with a VST for use in these applications. Running two standalone Pianoteq applications does not give as much control (by a long shot) as running it as VSTs under a DAW, but it's simpler. You could, for example, play both a D4 piano and the K2 together. A minor trick is that you can transpose one instrument differently and e.g. play an octave by striking one key.
ASIO drivers should not, I think, affect notes based on frequency. I don't think that's a problem.