I don't understand your choice of words to describe your situation.
Fidelity and Ameritrade don't give me a 5% discount. They simply participate in the company sponsored plan whereby the company issues new units at 95% of its formula price in exchange for reinvested distribution cash. ETP gives a 5% discount, not the broker.
Are you trying to say that UBS used to participate in ETP's discount drip plan (it is only a year old) but, without notice to you, they discontinued that participation?
If yes, I suggest you not go to arbitration pro se.
Would it be easier to fight - arbitration is your only option - and then move your account? Others will even pay you to move your money there! Am sure there is something in all those pages that gives them an out.