Hi Paul;
FWIW: Personally, I would not worry about PowerServer licensing beyond the trial period for your development environment. In PB2017R3 (revision #3) being released later this week, it will now include PowerServer Web and PowerServer Mobile with the Universal Edition at no extra charge. Also, the version of PS with R3 will now utilize the online licensing model controlled via the User Center like PB2017Rx already offers. Plus, off-line DB support on the Mobile devices will also be included with this PS edition!
The only extra PS licensing / cost consideration after R3 Universal development will be for production use of PS.
Food for thought. HTH
Regards ... Chris