I did not look into Theophilus' history til now, and it is a wow moment. There are numerous questions to ask - he left the Church of England in 1782 but was the sponsor for John HALL going to Chew Magna in 1788. How did that happen, the church record shows LINDSEY was at the Unitarian Essex St Chapel? Before HALL went to Chew Magna there was a court case between the widow of the previous Chew Magna vicar (Rev PYKE) and LINDSEY, I guess she objected to LINDSEY's recommendation, but the court found in favour of LINDSEY (Salisbury and Winchester Journal - Monday 30 July)
John HALL: born abt 1755, died in Chew Magna 20 Apr 1841 (buried 26 Apr), age at burial 84 (Gloucester Journal).
Married Sarah BLACKBURNE 14 Aug 1782 in Brignall, Yorks. Sarah died May 1811, Chew Magna (buried 11 May).
Children: John 1784-1784, Jane 1789-1789, Hannah Mary 1791-1844, Sarah 1794-?, Theophilla 1795-1839.
John HALL married Harriet Mary BALL 1 May 1817, bride from Wrington b.1769 d.1832
I've read several articles about Theophilus but have not matched any names to the John HALL side of the story, except Francis BLACKBURNE. It's worth repeating your observation that BLACKBURNE died in 1787 and John HALL may have lost some backing as a result.
I have not found anything to suggest that John HALL was in any way non-conformist.