Pwf has more to do with your wellbore and producing conditions than it does your reservoir. In your case, the '100 psi' Pwf is probably a minimum producing BHP that a water pump can maintain without burning up. As your reservoir pressure reduces, theoretically (operational limits permitting) you should still be able to maintain that 100 psi FBHP albeit with reduced water rates

As the coal dewaters and the Pr falls below the saturation pressure, gas will start being desorbed - ultimately (assuming minimal water ingress) you will be operating at quite low Pr, FBHP and water rates as you step down the langmuir sorption curve - At this point, if you had a 50 psi WHP, you could quite easily have less than 100 psi BHP assuming fully dewatered as most of these plays are relatively shallow.