Re: working in direct sun
For most products the reason not to use in direct sunlight is indeed because of the temperature of the panels. As we all know, the panels absorb the sunlight (particularly darker colours) and can end up being much much hotter than the ambient air. When you apply cleaning products, they basically boil on the surface. The biggest issue with this is that cleaning products are bunged full of non-volatile components and these will then be baked onto the surface. You thus end up with the component parts of the formulation on the surface and many of these are quite corrosive (when at the levels in the formulation, this danger does not exist). Further than that, many of these components, whilst water soluble, are going to be dehydrated to the point that rehydration is pretty tough - so they can be extremely hard to remove.
With polishes, this is perhaps not just as big a problem. Most polish products are designed to evaporate away and intentionally leave residues. In the real world you have to take a common sense approach. If the panel is a bit warm then you are probably fine, the polish may simple not 'go' as far. As panel temp rises, the work time will lessen so it may become unreasonably short and you will also get to the point that excessive levels of the film forming components are present and the finish would potentially be compromised as well.
Best to avoid if possible, if not - do small areas and have a water spray handy to keep your work area moist.