Assalamualaikum,
We have been told by Allah and His Messenger(s.a.w.) that we will have to answer for many things. The first action to be checked will be our Salah. If it is accepted as complete, then the rest of our deeds will also be complete, and if it is found lacking, the rest of our actions will also be found lacking.
Also, we have been given many Faraiz, such as Salah, Fasting, Zakah, etc. If we try to fulfill any of them with anything else, then that will not be acceptable. As such, Duroods will not replace Salah. That is, you cannot miss a Salah and say that you will fill it in by sending more Duroods on Rasulullah(s.a.w.).
Another point to note is that the "Sawab", i.e. virtue of anything is with Allah, and only He and His Messenger(s.a.w.) can inform us of it. Since neither the Quran nor any Ahadith tell us that someone who sends a certain number of Duroods will be forgiven everything, we cannot say that.
Wassalam
