What are the preferred temperature/humidity conditions for the slowrise process? I'm considering using it in my bakery for English muffins.
Less yeast and cold fermentation (fridge) is best to get those sugars converted in your flour.
You can use the temperature that works for you. Somewhere between 35 °F and about 50 °F. You will need to have the dough in a closed container or covered to prevent it from drying out. If you have a way of controlling humidity at these temperatures, keep it high.
By balancing the amount of yeast in the dough with the temperature in the cooler you should be able to make the timing of the whole process work for your schedule, whatever that might be. You can further control the process by altering the temperature of your ingredients at the time of mixing.
You could buy a "slow acting" yeast such as DCL EasyBake Slow Acting which allows you to do your normal process, it just takes allows a longer natural proof.