Are there other parts of a Lobster that are edible apart from the Tail and Claws

Restaurants in general use WAY more salt than home cooks do. I'd try increasing the salt and see what happens to the flavour. The same is often true of butter.

Also, some restaurants that publish recipes, do not actually publish something that is faithful to what they serve. I noticed it with a dish with which I was very familiar from a favourite restaurant. Certain ingredients were completely missing from the published recipe.

Edited to add: The sweetness diminishes with storage time and the meat from the smaller claw is sweeter and more tender. (reference) It's also possible that a particular type of lobster or size of lobster is more or less sweet. Maybe you need to get multiple fresh lobsters and use only selected meat for your roll (and do something else with the leftovers)? In On Food and Cooking, there is a suggestion that vanilla (normally used for sweet cooking) can be used when cooking things like lobster. It doesn't directly state what the vanilla does, but it's possible that it might increase the taste of sweetness. A bit of googling does turn up mention of lobster roll recipes with vanilla as an ingredient, so it might be worth experimenting.

This article indicates that it's probably gall bladder bile:

The gall bladder [...] sometimes [...] breaks even when you are cleaning the fish very carefully and close to the skin. If the gall bladder is broken, the greenish yellow biliary fluid pours immediately out into the inside of the fish and starts to absorb into the meat. The meat becomes bitter.

[...] see if the fish is greenish yellow on the inside or not. If it is, then quickly take a piece of paper towel and wipe it off; if it has already started absorbing into the meat, use a filet knife to cut off the part of meat that has turned yellow.

So, it looks like the gall bladder broke and stained some of the meat. If this is the case, cut the yellow bits away, and the rest of the flesh should be fine.