So according to some, each reading gets 3 blessings, and each reading is a discrete mitzvah with its own Shehechyanu at having reached the milestone of fulfilling that mitzvah. But some say that the day reading does NOT get a shehechyanu on it but hedge that position by having in mind time separated potential fulfillments?

@rosends No, those who don't think the day reading obligation is different enough from the night reading obligation to warrant its own shehechiyanu don't say one during the day at megillah reading. If they want to hedge an arguable need to say shehechiyanu on the other day mitzvot they would have to have them in mind at some other shehechiyanu (such as on a new fruit; it's debatable if they can have them in mind during the shehechiyanu at the night time reading, but I guess it can't hurt.)

My shul just sent out its bulletin in which there was a riddle about why we can't use a Shechyanu on the night of Sukkot (over sitting in the sukkah for the first time) to "cover" the mitzvah of shaking the lulav. The answer was "The time for the mitzvah of Lulav is during the day, and has not yet begun" but couldn't one say that the time of the daytime reading of megilla has not yet begun so a bracha over an evening reading can't apply to the daytime?

Indeed if they are two completely different commandments that logic applies (and I indicated in my parenthetical above that hedging from night to day is debatable). If it's the same commandment happening twice then it would be more analogous to lulav on the third day: even though a night interrupted you don't say shehechiyanu again since it's not the first time doing the mitzva.