To my knowledge, the exact role of the US and UK "observers" was never clarified. I fully expected them to stand by and watch their "charge" just walk away, as has been the custom in palistinian jails.
These prisoners may have been important to Israel, but I think the US negotiators considered them to be a minor point but a major obstacle. The main objective was to spring arafart -- a request that was personally delivered to President Bush by the Saudi Prince. The price for this deal was the US sandbagging the UN-sponsored "investigation" at Jenin. Powell could care less about these prisoers, who WILL walk IMHO.
Thanks for the clarification. I don't think it was worth it, unless there is more to it that we don't know. (probably)