Ok - these code changes did the trick! Looking at the datasheet again I see where I went wrong - I didn't read the 100 ON 900 OFF as a duty cycle for both on and off pulses. Rather, I had interpreted that to mean an ON pulse translated to voltage applied for 100ms then off (for no critical time) and an off pulse to be reversed polarity applied for 900ms then off. Any pulses duty cycle being 100 on 900 off makes perfect sense now, and seems to be working 100% as expected.
Thanks everyone!!