JED is run by a guy who used to be a principal at Allston, so he definitely knows what he's doing. They are a (mostly) algo shop with proprietary software.
Backtesting programs can account for slippage and commisions, so what difference it makes for them. My strategy can use market orders only and it won't affect the performance at all
Yes, there's certainly a range, but typically if you have a year of live trading with the strategy there's some people that will take a flier - assuming the returns are healthy enough.
It ends up being about optimization - they want you to tune the strategy over an extended period with real data, then bring it to them fully baked.