Is male pattern baldness guaranteed to progress to NW7 if left untreated or could it stop at NW5A on its own?

If you are losing hair and would like to understand what options are available it is best to see a doctor for an examination.Not everyone goes completely bald.  A certain pattern of hair loss is genetic. 

Hair loss is genetic and although exam and family history can help predict where you will settle out it may be hard to tell for sure at a young age.  I'm not sure exactly what you mean by a "thick low hairline", though it can be a mistake to place the hairline too low at a younger age as this may look less and less natural as time goes by.  Transplanting the front/hairline area and using a hair system for the crown is certainly reasonable.  You'd do best with an in-person consultation with a hair restoration surgeon for an evaluation and to discuss specifics as to where your hairline should go.

No, hair loss patterns of balding are each inherited. The Class 7 pattern is only inherited in 7% of men, so if you are a Class 6 you are probably going to stay that way. Hair systems accelerate hair loss as the glues and attachment mechanisms produce traction alopecia