What is the origin of your Fatwa, I mean the school of thought you belong and please advise me the way of invoking blessings upon the Prophet (Sallallahu Alaihi wa Sallam).

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

As for your first question, please read the Fatwa: 81802.
The Prophet (Sallallahu Alaihi wa Sallam) urged Muslims to invoke blessings upon him as much as possible. He said: "Whoever invokes blessings upon me one time, Allah will bless him ten times" . [ Ahmad ].
Imam Tirmizi narrated in his 'Jam'e' with a good chain as graded by al-Sheikh al-Albani from Ubayy Ibn K'ab that he said to the Prophet (Sallallahu Alaihi wa Sallam): "O Messenger of Allah! I invoke Allah's peace and blessing upon you copiously; how much time of my prayer should I spend for this?' He said: 'As much time as you think proper'. I said: 'A quarter of my time?' He said: 'As much as you wish; but it would be better for you, if you could devote more time' I said: 'Half of my time?' He said: 'whatever you wish; but it would be better for you if you were to increase it'. I said: 'shall I devote all the time of my prayer - i.e. shall I supplicate only for you, not supplicating for me - He said: 'Then it will take care of all your worries and your sins will be forgiven" .
The best words for invoking blessings upon the Prophet (Sallallahu Alaihi wa Sallam) are the words he taught his companions when they asked him about it. Imam Muslim and others narrated in Sahih Muslim from Abu Masoud al-Ansari who said: "We were sitting in the company of S'ad Bin 'Ubadah when the Messenger of Allah (may peace be upon him) came to us. Bashir Bin S'ad said: 'Allah has commanded us to bless you. Messenger of Allah! But how should we bless you?' He (the narrator) said: The Messenger of Allah (may peace be upon him) kept quiet (and we were so much perturbed over his silence) that we wished we had not asked him. The Messenger of Allah (may peace be upon him) then said: (For blessing me) say: "O Allah, bless Muhammad and the members of his household as You did bless the members of Ibrahim's household. Grant favours to Muhammad and the members of his household as You did grant favours to the members of the household of Ibrahim in the world. You are indeed Praiseworthy and Glorious" .
Allah knows best.