Based on your description of the scenario it looks like you are being painted as the escape goat. They have absolutely no proof that it was your unit. And to rack up a $1500 water bill from a faulty toilet seems unlikely. I would go as far as calculating the refill rate of your toilet tank and see if it is even possible to use that much water in a month's time, which I have.

The average water rate is $1.50 for 1,000 gallons in the US, let's say it takes 1.6 gallons per flush. I happen to have a 1.6gpf toilet and takes exactly 30 seconds for the tank to refill. For my tank to use $1500 worth of water @ $1.50 per 1000 gallons, it would take: ($1500/$1.5) = $1000 x 1000 gallons = 1 million gallons / 1.6gpf = 625,000 flushes x 30 seconds refill rate = 18.75 million seconds = 217 days.

It take my tank 217 days to rack up a $1500 water bill if it had a leak, now let's say you had a 3.5gpf tank and same refill rate, highly unlikely, it would sill take 99 days to rack up a $1500 water bill. So based on that it ain't a leaky toilet.

As far as I see it you all bought the units and agreed to share a water bill. You did not intentionally flush water down the toilet, sorry I couldn't help my self, just to piss everyone off. You sent your plumber to check on it and they found nothing. I could have easily been any other unit. They have no proof what so ever that it was your unit. I would tell them that I would take responsibility if there was proof that it was my unit, but until there is proof, then fuck off, as nicely as I can of course.