So, you didn't explain how he could do that. Last time it took a spell from Quirrel to hold the shell in place. I'm guessing it's not as simple as holding the shell with one hand and spinning the hourglass inside it.

Quirrell anchored the hourglass with a spell, and then Harry spun the shell around it. We never see Harry perform the anchoring spell, but we don't have any information about its difficulty, so presumably Harry could do it too if he prepared.

Also, Harry can't clone (loop) himself four times during the trial if he goes back two hours. That would result in looping himself four times for the time period from 2 to 1 hours before present. To loop himself four times (or he could make it five) during the trial itself, he should go back only one hour.

He's giving himself an extra hour to prepare. The second, third, and fourth iterations only involve going back one hour, so that leaves a turn to spare. I'll edit to make that clearer.