Considering the high amount, and the long duration of the escrow, you may want to consider a 2-out-of-3 multisig: OP holds 1 key, and 2 escrows each hold one too. That way, 2 out of 3 parties can always make a transaction.
That's a good idea. Two trusted escrow sources who don't normally work with each other.
Also, I don't know why you would offer more for the escrow service if the bounty succeeds as opposed to not. Lets be honest there are plenty of places on this planet that the perp's IP could be traced to that could result in a dead end. If you're serious about attracting a bounty hunter, then you have to be serious about attracting an escrow, and therefor you must be willing to pay, whether the perp is found or not.