It's true that they did make amazing shots, some akin to Tatum game 6 in Milwaukee. Miami making those shots is not what I found so frustrating about the Celtics performance yesterday.
You are right about the Horford thing. In Q3, Rob played 10 minutes, scored 10 points, mostly if not entirely as a lob option late in the quarter. With 5:30 in Q3, the Celtics were down 70-58. They outscored Miami 17-12 over the rest of the quarter to cut the lead to 7, and it was all Rob (10 points) and White (7 points). Rob never saw the floor again, though he was up at the scorer's table once and Ime called him back. And the lob option worked phenomenally well in the third, and as the Celtics' offense imploed in the 4th it was never an option. Maybe there were health reasons why Ime did not want to use Rob unless he has to, but that aside - and it seems doubtful at a glance - it was a costly coaching mistake.
To make shots, one must take them. Tatum and Brown took 7 shots combined in the second half, each had only one in the fourth quarter. Tatum had 2 made threes early in the third, then he missed a three at 8:14 of the third. From that point until the end of the game he had only one more shot from the field, despite being on the floor for the entire second half. In the 4th, he turned it over 4 times. That's not "top 5 player" work. Can't be a top 5 player AND be essentially a passnger for most of the second half of a key playoff game (even if the opposing D gets credit for the latter). He has a ways to go in his development - maybe it is too much to ask of player to figure this stuff out in the middle of a playoff run as opoosed to over the following offseason. Butler found ways to score at will.
Brown, who had a great second quarter (11 points and 3 steals), misse his only 3 shots of the second half and missed both FTs with the Celtics tied 99-99, right before the wheels fell off.
Rob Williams, whose career regular season FT% is 67%, has somehow morphed into the Celtics most reliable free throw shooter in the playoffs. He's 18 for 20 from the line in the playoffs, 15 of 16 in the Miami series, and has hit his last 10.
I think they need more from Tatum than one shot from the field taken in the final 20 minutes. If that is not on Tatum it has to be on Ime or others on the team. Clearly, Spoelstra is winning the coaching battle in this series, though I suppose that is to be expected with a promising rookie going against one of the best in the league.