1. This is related to the richness of the gas in heavy ends - its not something you can calculate from properties such as viscosity (although I'm sure there is some relationship). You really need to have some understanding on how rich or lean the gas is likely to be
2. Using wellhead abandonment pressure, and correcting back to the sandface pressure taking into account friction and head is an appropriate way of dealing with this
3. If you need to be able to incorporate number of wells required and timing etc, then you need to step up the complexity of your model. The number of wells required is related to not only individual well deliverabilities, but also structure size and continuity. The simple rate potential equation is suitable if you are modeling the field as a tank with one (set of) offtake(s).

General (simple) equation for well deliverability is rate = C(Pr^2 - Pwf^2)^n. You'll have to find your own values of C and n depending upon your reservoir