1) Is it proved by Sunnah anywhere that when we greet our relatives on Friday by shaking their hands and on Eid days when we greet them Eid Mubarak, We have to say ‘Allahumma Salli Ala Muhammadin Wa Ala Aali Muhammadin Wa Barik Wa Sallim’, i.e., Durood, or is it an innovation?
2) Is giving Khatam (reciting of Surah Fatihah, the 4 quls and half of the 30 juz) permitted on food on the death anniversary of a person. If not is such food Halal to eat? And is the practice of reading Surah Yasin a certain number of times and reading Durood and Qur’an certain number of times and passing on the reward of it at the time of Khatam permitted?
3) Is eating of Clams, Crabs Permitted? Referring to a Hadith in Sunan Abu Dawud as given below:
Narrated Jabir Ibn Abdullah: The Prophet (Sallallahu Alaihi wa Sallam) said: “What the sea throws up and is left by the tide you may eat, but what dies in the sea and floats you must not eat.”

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

1) Invoking blessings on the Prophet’s (Sallallahu Alaihi wa Sallam) like saying: “Allahuma Salli Ala’ Muhammad” or any other form of invocations is lawful, even if the form used is not reported.

But it is better to limit oneself to the invocations that were reported from the Prophet (Sallallahu Alaihi wa Sallam).  The best of those invocations is the form known as Al Ibrahimya.  But the form you mentioned in your question is a good one.  But we do not know any text that proves that this form is to be said when meeting relatives or greeting them on Fridays or feast days.  Saying it at these times can be considered an innovation, Bida’a; the best is to follow the guidance of the Prophet (Sallallahu Alaihi wa Sallam) and the worst is the Bida’a (innovation in religion).

2) It is lawful to read/recite Qur'an with the intention that its reward reaches a dead person be that Al Fatihah, Al Mu’awwathatain or any other verse of the Qur'an.  But specifying a given Surah like Al Fatihah, or a given time after the death for recitation is not acceptable in Shariah.

It is also lawful to feed some people and send the reward of that deed.

As for reading Surah “Yasin” next to the body of the dead person, the Hadith that states this is weak.  But some scholars are of one opinion that this is a good act because weak Ahadith can be used as evidence with some conditions as mentioned in the Fatwa 87179.

As for invoking blessing on the Prophet (Sallallahu Alaihi wa Sallam) on behalf of the dead or reading Qur'an, this is good.  But specifying a given time, place or state that are not specified in Shariah is unlawful as it is but an innovation that should be avoided.

3) The scholars of Islam have different opinions regarding crabs.  The Hanafi are of the opinion that eating crabs is unlawful.  They (the Hanafi) claim that from all the creatures of the sea, only fish is lawful.  Al Kassany (a Hanafi scholar) said: 'All creatures of the sea are unlawful to eat except the fish if it is not floating (after death) on the water'.

The Maliki believe that eating crabs is lawful.  Al Kharshy (a Maliki scholar) said: 'And any sea animal (is lawful) even if it stays for a long time on land.  This means for them that the Meytah of sea animals (dead without being slaughtered) is lawful for us to eat regardless of whether that animal was killed or was found dead.  There is no difference in this rule between an animal whose life on land is long like the sea turtle, the sea frog, or the crab, or the one whose life on land is short like the whale'.

The Shafi’e have two different opinions but their most preponderant opinion is that it is unlawful to eat crabs.  Al Nawawi said: 'The frog and the crab are forbidden and this is the opinion of the majority of scholars'.

The Hanbali are of the opinion that the crab is lawful.

Ibn Qudamah said: 'Imam Ahmad said, ‘There is no objection to eating crabs’.  Someone asked: ‘Should it be slaughtered?’  Imam Ahmad replied: ‘No’'.

As for the Hadith you mentioned it was reported by Abu Dawud from Jabir Ibn Abdullah in these words: ‘Whatever the sea throws away or leaves out during tides, eat it but do not eat what dies and floats on it’.

This is the evidence used by the Hanafi to prove their opinion.  But the majority of scholars are of a different opinion for the general meaning of the following Hadith: "Its (the seas Meytah/dead animal) is lawful". [Ahmad].

The scholars believe that Jabir’s Hadith is Mawqoof i.e it is his saying, not the saying of the Prophet.

As for clams, they take the same rule as crabs.  So, although both are permitted according to some scholars, they are not according to others.  So, we believe that the safest is to avoid them.

Allah knows best.