Well, I thought the whole idea is that you DON'T HAVE TO trust the escrow providers.
In the CoinGuard case, it seems that each party will hold some secret to themselves and the escrow provider (E) is only needed if the buyer and seller have a dispute. And the escrow provider cannot take the BTC funds, except to give it to either the buyer or the seller. That's how I understand it works, not sure if the CoinGuard web site explained this clearly.
With regard to the GGDice service, I think it's theoretically possible that the private key is stored on the server, that would be a critical flaw in the scheme, to me anyway.
This is correct. However, both CoinGuard and GGDice seem to create the keys on the server. This makes the service easier to use, but of course has a lower security than the 'raw' approach of Alice and Bob creating a key separately on their own computer and then sending both public keys to the other.
Just like with any site, behind the scenes your data could be saved/used/manipulated in any way. But this is true for every kind of website. So, a certain amount of trust/good faith is always neccesary when filling in any form or sharing any information with anyone.