Not entirely clear. Church historians note that Calvin began his Bible study at John Beach’s house before the chapel was finished, but it doesn’t appear that the Stowes relocated to Hartford until 1864. The earliest reference I’ve found to them living in Hartford is May 1864, and they were definitely living in Hartford by December 1864. These dates suggest that dates for the Bible study starting in 1861 or 1863 are too early. The Asylum Hill Congregational Society formed in June 1864, and the chapel was finished in March 1865, which makes it more likely that Calvin became involved between June 1864 and March 1865.
It does not appear that he did. Calvin was not among the 114 founding members, and there’s no record that he subsequently became a member. In fact, during the memorial service for Calvin Stowe at AHCC, the local Presbyterian minister stated that Calvin had asked him specifically to state that Calvin had died a Presbyterian in good standing.
It does not appear that she was. Harriet was not among the 114 founding members, and there’s no record that she subsequently became a member. Unlike her husband, Harriet did not receive a memorial service at AHCC, but Joseph Twichell did participate in the funeral held at her home on Forest Street. Also, while AHCC rightly claims Samuel Clemens as a member, the church never makes a similar claim about Harriet.
Very, in Joseph Twichell’s opinion. Twichell considered Calvin a friend and mentor, and he cited Calvin’s Bible study as a critical part of the church’s formation. It remains unclear, however, when exactly Calvin became involved, or even how he became involved.