I think we can presume that last flashback was not long before Isshin and Masaki met, in fact it wouldn't surprise me if that's where he's off to at the end there. If so that would mean this is roughly 25 years before the start of the series.
I honestly think he's had this Quincy thing planned from really early on. The relationship between Isshin and Ryuuken now makes perfect sense, brother in law through the marriage to Masaki. That would also explain the Ishida and Ichigo appearance similarity. As well as that there's the whole Aizen being interested in him from before he was born, I imagine shinigami and humans have had kids before but the idea of knocking up a Quincy is probably considered worse than bestiality, thus an interesting way to kill time. The
first interaction between Ichigo and Aizen also shows his interest, If he wanted to cut him in half he would have cut him in half because he's Aizen.
Oh, and I seem to remember reading that Ichigo respects William Shakespeare. Inb7 this while Misaki x Isshin thing plays out a little like Romeo and Juliet.