You will probably be much more comfortable in a breathable fabric than a waterproof. If a pad works for you in a single layer hammock I can't see a reason for double layers, unless you are using a light fabric that might tear easy in a single layer, or if you are worried about mosquitos biting through the fabric and don't want to use/can't get something like permethrin to treat it. Most people find a pad in a double layer hammock easier to manage and more comfortable as you don't have to lay on it directly, and the pad will stay put better.

I don't know what sympatex is, so can't comment on it. But a fabric that does not stretch under you and holds your weight without tearing will work. Most hammocks made for camping are nylon ripstop of 1.1 or 1.7, some with heavier nylon. 2.6 oz per yard sounds heavy but if you are not backpacking maybe that is not a concern.

You have very good English.