Question: Who was perhaps not the only king of the West Saxons at the time, whos viceroy is said to have constructed Chisbury Camp, and to have founded Abingdon Abbey?

Context paragraphs:
Æthelberht, King of Wessex: Æthelberht (or Ethelbert; Old English: "Æþelberht" , meaning "magnificent noble") was the King of Kent from 858 and of Wessex from 860 until his death in 865.  He was the third son of Æthelwulf of Wessex and his first wife, Osburh.  In 855 he became under-king of Kent while his father, Æthelwulf, visited Rome.  His brother Æthelbald was left in charge of the West Saxons.  After his father's death in 858 he succeeded him as king of Kent and the other eastern parts of the kingdom.  When Æthelbald died childless in 860, the kingship of the West Saxons also passed to Æthelberht.

Centwine of Wessex: Centwine (died after 685) was King of Wessex from c. 676 to 685 or 686, although he was perhaps not the only king of the West Saxons at the time.

Abingdon Abbey: Abingdon Abbey was a Benedictine monastery also known as St Mary's Abbey located in Abingdon, historically in the county of Berkshire but now in Oxfordshire, England.

Historia Ecclesie Abbendonensis: The Historia Ecclesie Abbendonensis or History of the Church of Abingdon (sometimes known by its older printed title of Chronicon Monasterii de Abingdon or occasionally as the Abingdon Chronicle) was a medieval chronicle written at Abingdon Abbey in England in the 12th century.

Battle of Bensington: The Battle of Bensington was a major battle fought between Mercia, led by King Offa, and the West Saxons led by Cynewulf of Wessex.  It ended with a victory for the Mercians, and the West Saxons recognizing Mercian overlordship.

Cissa (West Saxon): Cissa was reported as the viceroy of king Centwine of Wessex (reigned c.676-686).  Cissa is sometimes said to have himself been a king of Wessex, but does not feature in the king lists or genealogies.  He is said to have constructed Chisbury Camp, and to have founded Abingdon Abbey.

Æbbingas: Abingdon and Abingdon Abbey were founded in Saxon times, possibly the 7th century.  The early history of Abingdon (and its abbey) has been distorted by the numerous legends surrounding its history.  The legends were invented to raise its status and explain the place-name.  The name seems to mean 'Hill of a man named Æbba, or a woman named Æbbe', possibly the saint to whom St Ebbe's Church in Oxford was dedicated (Æbbe of Coldingham or a different Æbbe of Oxford).  However Abingdon is actually in a valley and not on a hill.  It is thought that the name was first given to a place on Boars Hill above Chilswell, and the name was transferred to its present site when the Abbey was relocated.

Ælfric of Abingdon: Ælfric of Abingdon (died 16 November 1005) was a late 10th-century Archbishop of Canterbury.  He previously held the offices of abbot of St Albans Abbey and Bishop of Ramsbury, as well as likely being the abbot of Abingdon Abbey.  After his election to Canterbury, he continued to hold the bishopric of Ramsbury along with the archbishopric of Canterbury until his death in 1005.  Ælfric may have altered the composition of Canterbury's cathedral chapter by changing the clergy serving in the cathedral from secular clergy to monks.  In his will he left a ship to King Æthelred II of England as well as more ships to other legatees.

Colne Priory: Colne Priory at Earls Colne, Essex was a Benedictine priory, initially a dependent cell of Abingdon Abbey, Berkshire (now in Oxfordshire).  It was founded by Aubrey de Vere I and his wife Beatrice in or before 1111.  Their eldest son Geoffrey had died at Abingdon about seven or eight years earlier and was buried there.  On his deathbed, Geoffrey had bequeathed to Abingdon the church and lands at Kensington, Middlesex, and his parents and brothers had confirmed that grant, as had King Henry I.

Miles Salley: Miles Salley (died 1516) was a late 15th-century Abbot of Eynsham Abbey and Abingdon Abbey and an early 16th-century Bishop of Llandaff.  Salley was Abbot of Abingdon, followed by Eynsham in Oxfordshire in the 1490s, during which time he was party to a major quarrel with Sir Robert Hartcourt of Stanton Harcourt which led to violent clashes between the monks and Harcourt's servants.  In 1500, he was appointed Bishop of Llandaff, where he is remembered for his building work at the Bishop's Palace in Mathern in Monmouthshire.

Supporting paragraphs: Cissa (West Saxon) Centwine of Wessex
Answer: Centwine