What acts of worship can be performed by proxy, and which cannot? And is it permissible to combine the intention of four rak'ahs of Sunnah prayer before Dhuhr with the two emphasized ones?

It is not valid to combine two independent acts of worship, each intended for its own sake, with a single intention. Rather, it is permissible to combine an act of worship intended for its own sake with another that is not intended for its own sake, as they can overlap. Therefore, praying two rak'ahs before Dhuhr does not suffice for the four rak'ahs of the Sunnah prayer (ratibah), because each of them is independent and intended for its own sake.