If JetBlue were to launch to Edinburgh, they will no doubt take some passengers from existing services, but at the same time they tend to be a bit cheaper than the competition, which will stimulate additional travel demand too. Most US flights tend to go out quite full at the moment anyway, so there's probably excess capacity there to be picked up. I think there's still room for an additional carrier on the route.
In terms of West Coast flights - Scotland is on the wishlist for LAX, so they certainly thinkt here's demand and are keen to attract someone on to the market. Sadly there arent many candidates for the route. Virgin in partnership with Delta might be the only possibility. It's a shame Barbados didnt work out. If that had succeeded, then they may well have looked at basing an aircraft at EDI and offering LAX a few times a week. Who knows... if they can perhaps talk LAS in to doing incentives too, they may decide it worthwhile to base an aircraft at EDI and operate to both 2x weekly with a 3x weekly Orlando flight, or similar.