1. Basically, yes, reflectance panels are for multispectral data because you need to convert DN to reflectrance.
2. There are two sets of parameters to determine: spectral and directional. Real life reflection is described by a bi-directional function (
https://en.wikipedia.org/wiki/Bidirectional_reflectance_distribution_function ) which means that parameter set depends on aircraft direction. Theoretically, you can measure all the values and calculate the corrections, but it is a hard task.
3. Well, you may use white/gray panel to determine light conditions at the moment the picture was shot. And this may be applied for pictures with same light conditions. But, once again, due to directional component, IRL you have more than two parameter sets (not only "sunny" and "cloudy").
4. Actually, you don't need color panels to set WB, you need only white/gray card. And in my opinion, some cameras may benefit from custom WB in JPG mode, even expensive ones. Auto-WB works really simple: camera averages whole image and adjust multipliers so that average tone is neutral gray. Sounds coarse, isn't it?
I'd suggest you to try several flights with fixed WB (not necessarily RAW). It may improve overall color situation under constant light conditions.