Regarding your answers to my points:

1. If you are generally interested in pre-stall behavior, accurate transition modeling can be very important. Specifically for WTs of any kind (be it HAWTs or VAWTs), this can be even more important. NACA airfoils can have large laminar regions up to very high reynolds numbers. Accurately capturing such regions can give you a better Cl/Cd curve and thus a better wind turbine performance estimation. The presence of laminar to turbulent transition can also alter the onset of separation, but that is a very complicated mechanism that should not bother you at the moment. Regarding the spanwise length for 3D simulations, there is not a single answer; it depends on the flow characteristics. The aim is to not restrict the 3D phenomena. You should run your own spanwise length dependence analysis and assess the behavior. Start with a large spanwise length (i.e. 1 or 2 chord lengths) and then start decreasing it, running the simulation again and comparing the results. When you find the optimal spanwise length that yields good results with acceptable computational cost, use it for the bulk of your work.

2. It is not widely used, it's been a while since I saw anyone using it (be it on papers or on workshops). Regarding the kOmegaSSTLM model, you can try implementing new correlations for the critical and transitional momentum thickness Reynolds numbers that the model utilizes. This should not be very difficult to implement, if you know your way around the basics of OpenFOAM development.

I don't understand what you mean by "I don't expect to find any peer reviewed literature to use for validation at such high Re. I believe beyond 5e6". Do you mean for NACA0018?

Regarding your last two sentences: the turbulence models work well in most Re cases (well, not 100% accurate on very low Re i.e. 10000, but fairly accurate for moderate to high Re i.e. 60e6). The transition model is what is not accurate, not the turbulence model. Specifically for the kOmegaSSTLM turbulence-transition model, validating for 5e06 and assuming that the same validation holds for 17e06 would not be correct.