Thanks everyone for your advice. I've decided to spend a half day in Sendai and leave in the early afternoon for Tokyo.
From Tokyo to Nagoya, I'll stop off in Odawara for about 2.5 hours to eat lunch and see the castle.
Looking at the Nagoya-Osaka options, it does seem that the stopovers (except Kyoto) would involve a lengthy diversion and that I'm better off spending a half day more in Nagoya, going straight to Osaka, and then I'll head back to Nara from Osaka the next day.
I'd like to get to Osaka by early to mid afternoon to check into my hotel and then head to Kobe for the baseball game at 6 p.m.
Nagoya-Osaka-Nara-Osaka isn't much longer on the train than Nagoya-Nara-Osaka as far as I can tell.