And I assume they didn't loop again at the end, because Usagi didn't do the spell in the last loop.I've come up with my own theory as to this.
Since we know that some weird supernatural power is in play in this whole "absolutely will come true spell", it could not be wrong that some type of paradox is in play due to the overlapping spells of Usagi's and Tora's on eachother.
As for Usagi's death, it may have to deal with the last part of Tora's spell "may Usagi not belong to anyone," as every time Usagi tried to confess to Tora, she would be killed.