1-Can we say (YA NABI SALAAM ALAIK) "O Prophet of Allah, Assalamu Alaik"2-Is it true that Prophet will reply to our salaam?3-Can we give sadaqah like reciting Al-fateha for the dead people?

Praise be to Allah, the Lord of the Worlds; and blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

1. Muslims are asked to invoke Allah to bless and greet the Prophet. It is sound in Sunnah that the Angels inform the Prophet of each and every greeting that Muslims say and he replies to all those greetings. Abu Dawood reported in a Hadith narrated by Ibn Abbas that the Prophet said: "Allah has some touring angels on earth who convey to me the greetings of my Ummah". [Reported by a sound chain].
Abu Dawood also narrated in a Hadith that the Prophet said: "Every time a person greets me, Allah brings life to me until I reply to his greeting".(Reported by a good chain).
Know that if this invocation is used for "Istighatha" (seeking help) it is Shirk, polytheism and therefore should be avoided completely.
2. The majority of the scholars are agreed that the Muslim can do any good deed and send its reward to a dead Muslim person and that this reaches that dead person and helps him (Inshallah). But one should know that many of the things that people do nowadays such as reading some Qur'an on the grave or reading the Fatiha or the like were not known to have been done by neither the Prophet nor his companions. So, doing them is considered as an innovation.
Please refer to fatwa no:82681.
Allah knows best.