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Do you have any idea why they would entitle him "Master" in this will? a priest?

That seems likely, especially as both he and Master William Nicholl are given bequests in the interests of saying prayers.

Translation
[marginated] The Will of Richard Pomeray
In the name of God Amen. On the 24th day of the month of August in the year of the Lord 1496, I Richard Pomeray, knight of the diocese of Exeter, being whole in mind and of healthy memory (let the Most High be praised), make my testament in this manner. First, I leave my soul to God Almighty, and my body for sacred burial to be buried in the parish church of the Blessed Mary of Berry Pomeroy. Item, I leave to the same parish church ten marks sterling. Item, I leave to the parish church of Weare Giffard five marks sterling. Item, I leave to Thomas my son one hundred pounds in money, to be handed over to him after my death, and half my jewels after the death of Elizabeth my wife. Item, I leave to the vicar of the parish church of Berry Pomeroy for forgotten tithes, 40 shillings. Item, I leave to any domestic servant his wages for a whole year after my death. Item, I leave to the Prior and brothers of the Dominican convent of the City of Exeter a goblet of gilded silver and five marks in money, to pray for my soul. Item, I leave to the cathedral church of Exeter five marks. Item, I leave five marks to the abbot of the Abbey of Tavistock, and to the convent of the same place another five marks. Item, I leave to Edward my son a chain of gold. Item, I leave to Thomas my son another large chain of gold. Item, I leave to Master William Nicholl my farm that I recently had from Agnes my sister, and to the same Master William I also leave 40 shillings, so that he should pray for me and my relatives. Item, I leave to Master John my brother 26 shillings 8 pence, to pray for my soul. Item I leave to Blanche my daughter, her son, whom I recently held from the death of his father, with his lands and tenements, until he will have reached his lawful age.* Item, I leave to Elizabeth my daughter twenty pounds sterling. Item, I leave to the abbot and convent of Torre 40 shillings. Item, I leave to the abbot of Buckfast 40 shillings. The residue of all my goods not bequeathed above I give and bequeath to Elizabeth my wife, whom I ordain, make and constitute my executrix, so that she should dispose them for the health of my soul; and I ordain and make as supervisors of this my will John Sapcott, knight, and John Kirkeham, esquire. And I will that the same John Sapcott should have for his labour and diligence one hundred shillings; also I will that the aforesaid John Kirkeham should have for his labour and diligence five marks sterling.

     * - it would appear that Richard had the guardianship of his grandson while the latter was under age.

The abbreviations in the Observations column are the individual parishes within the St Saviour's Union to which each pauper was chargeable, e.g.
N - Newington
SG - St George (Southwark)
SS - St Saviour (Southwark).

Champion Hill (in East Dulwich, opened c. 1887) was the Infirmary site for the whole St Saviour's Union, nothing to do with mental health.

See here - scroll down...
https://www.workhouses.org.uk/StSaviour/#Post-1834

Essentially, the postscripts show that on 23 May 1646 the son Francis Stebbyng asked the court to let him have the original will back, proving his entitlement to the greater part of the lands and tenements that had been left him in his father's will. The Chancellor, Clement Corbett, issued a decree in favour of this.

An exact copy of the will was then made and cross-checked with the original. The court would keep this copy, and when Francis returned the original into court it would be checked against the copy to ensure that nobody had tried to alter the original. This was standard practice.

The signatures are simply to confirm and witness what had been agreed. (Turfett is the Notary Public.)

... the MP is Sir Thomas Travell.

Good call. The Tr... is easy to see (once you know!).