Jerry,
Wow, a very good listing of risks.
At this moment, I am already diverifying, I have started using 2 brokers and I added USD/JPY to my GT-Shadow currencies porfolio.
The most serious points seem:
2. They can really kill RB/GT-Shadow profitability with swap rates. This is already a problem with long standing positions and with somebrokers...
5. This is already happening I think that FIFO has been instituted because someone wanted to drive a whole swarm of robots off the US (so we got a precedent).
On the other hand, RM trades should not bother the brokers too much, because they do make profit on them on spreads that are quite high compared to other major currency pairs. Every closed RM trade means 6-20 pips spread in their voluminous pockets. I think however that if the big fishes smell that grid trading is really profitable they start using it themselves -- and start lobbying against using it by the public...