“The rider shall normally assume a sitting position on the bicycle. This position requires that the only points of support are the following: the feet on the pedals, the hands on the handlebars and the seat on the saddle.”
It doesn't say you have to use all of them all of the time. Otherwise it'd be contrary to the rules to ride one-handed, which would make refuelling and hydrating a wee bit tricky.
No it's not. You cant use the top tube as a replacement contact point for your arse, doesn't mean you cant use no contact point for your arse.