From what I have read Otsu was presented as Musashi's vision of idealized, perfect, chivalrous love in the original novel. They part ways as teenagers, never consummate their love, never argue, never fight, and never have to see the other grow old. It is a vision of "pure", courtly love, unhindered by the wickedness of the flesh as only a 17th century samurai or medieval knight could imagine it. Musashi can hold Otsu up as an ideal, as the principle of what love should be, without the dirty business of reality clouding his vision. I do not think Inoue has really tried to portray this, but from what I know of the real legend, that is Otsu's role.
Musashi seems to regret abandoning Jotaru, it was certainly on his mind before he decides to stay with Iori. At the time he met Jotaru he would have been a horrible surrogate father, dragging a boy along to some incredibly bloody duals. Imagine the escape from the Yoshiaki if Jotaru was in tow. At this stage in life Musashi seems more interested in settling down than proving his manhood. I suspect Musashi will give up on trying to tend that ruined field, and instead seek employment as a retainer, as an actual samurai, bringing Iori to the relative safety of a lord's castle. Musashi may recall that in their last conversation,
Takuan offered him the position of sword coach to the Tokugawa shogunate in Edo.
