OP - Make a claim to UA for 400% of the value of the segment, capped at $1,350 (the current maximum). Keep the complaint to 3-4 short declarative sentences. All that matters is that you were checked in prior to T-30, and at the gate by T-15 (UA's published minimums), that you had been off-loaded and the flight closed. That's all that matters. The rest is all fluff. What the cop, the TSA guy said, whether the GA was rude --- none of it matters. UA can see when you checked in and when you were off-loaded.
If you are compensated, all to the good. If you are not, pursue the DOT complaint and see what happens.