To answer this, I need to clarify where and how I have worked with sugar. There have been two types of locations: sea-level at reasonable humidity rates (50-100%), and high altitude (5500 ft +) at low humidity rates (less than 15%) and at reasonable humidity rates (50%+). I would love to try working sugar in a low humidity/low altitude location, I just haven't been in the right place at the right time to try it. Based upon these different locations, here is what I have experienced:
From a crystallization point of view, I have not seen much difference in avoiding crystallization at different humidity levels. The techniques to avoid crystallization (use of glucose in the recipe, careful when agitating, etc) are the same. I have seen a difference in timing and temperature due to humidity and altitude changes to reach a desired caramel/sugar consistency, however.
For instance, when working at my current altitude versus sea level, and at low humidity levels, I reduce the temperature to which I cook the sugar solution by 8-10 degrees versus what I would do at sea level at higher humidity. If it is humid here (which is sometimes is during monsoon season), I only reduce the temperature by about 4-6 degrees. This indicates that both factors are a component. As a specific example, right now at 10-15% humidity and 5500 feet in altitude, if I want a soft ball stage caramel I will cook it to 228-230 degrees F. In July or August here the humidity will jump to 80-100%, and I will cook the same recipe to 232-234 degrees to hit the same viscosity. At sea-level, I would have taken the recipe to 238 degrees to hit the same. This convention (come across by great trial and error), holds true for any of our sugar based confections: fondant, caramel, toffees, nougats, pulling sugar, etc. We hit the standard sugar stages at lower temps at low humidity levels. My assumption for the reason for this is that at lower humidity rates and higher altitudes the rate at which the water evaporates from the supersaturated solution is higher at each degree of temperature than at higher humidity levels, meaning that we are losing water from the caramel at a faster rate and reaching the appropriate water/sugar ratio for a given stage earlier. Note that this is merely conjecture, but it is what I am seeing in practice.
Back to the question of alcohol, upon reflection, every once in a while I will add alcohol (particularly a wine that may have residual sugars and adversely impact the desired flavor of the caramel) during the cooking stage, but only after the solution has come to a boil. I have not had any crystallization issues with this, although it does seem to also speed up the stage levels but I have not experimented enough to come up with a quantified effect.