Apart from the Church/Theatrical references (both Bishop Usher and the show-to-seats versions), 'Ushing' was IIRC the technique of washing heavier metal ores (such as lead) from the general spoil, a little like panning but using continual agitation on a fine sieve.
Perhaps 'Usher's Well' started as 'The ushers' Well' in a lead mining region.

Just a quick two penn'orth.

Walrus.