Meeting with a SS Coach is going to do more for you than anyone can across the wires. With that in mind, I can tel yo that consistency builds progress. Whether you work out two or three times a week is less important than continuing to do it week after week.
I would not call the downsides severe. Three is better, but do what you can.
Rip has mentioned on a few occasions that back extensions are not critical. Don't worry about them if you don't have the equipment to do them.
Probably. However, it is premature to worry too much about that at present. What are your gender, age, height, and weight? If you are a male, you likely have a lot of workouts ahead of you before you need to worry about changing anything.