In reply to:

When and how did English lose the 2nd person singular?

As to how, it just stopped being used, replaced by the plural, except that the nominative "Ye" was replaced by "You". As to when, it happened in the first half of the 17th century in England. It was still in regular use at the time the King James Version of the Bible was produced (1611), but you have to bear in mind that that work was produced by mature men who, presumably, spoke and wrote the language they learned in the last quarter of the 16th century. But by the 1620's, you find it being used less, and by the Restoration period (1660's), it's not being used regularly except for poetical and liturgical purposes. In fact, you can track the process to some degree in the work of a single author, John Donne, in his poems, letters and sermons. (By the time he died, 1630, he was not using the singular regularly, although he did in his early works.)