I think that 'by which point' would be considered a phrasal subordinating conjunction (= 'when', 'at which time'), but there are probably other ways of analyzing it.
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English 1b3'By which point' must work as a relative clause or something.Correct. The whole clause starting there is a nonrestrictive relative clause.
English 1b3Can you please explain why how that is not a second main clause, starting from 'by'?Because that portion of the sentence can't stand alone. If it did, it would
English 1b3If it were "by that time" or "by this time", it would not be a relative clause, correct?Correct. that and this are not nonrestrictive relative pronouns.