Having read Act 17 and act 18’s preview synopsis, I have a theory about how this all works together.
I think the arranged marriage thing is correct and that the arrangement is for Mamoru and that girl to be married after graduation, which is probably about 6 months from now. The phone call could have been the person who arranged the marriage trying to get Mamoru to move the wedding date up. During the call, Mamoru seemed quite shocked and defensive as he reminded the person he had until graduation. Whoever it was, they obviously have some kind of leverage with Mamoru to be able to make him do this when he's obviously reluctant.
Remember back when Sailor V told Sailor Moon to stay away from Tuxedo Kamen? Sailor Moon said she couldn’t and Sailor V mused something to the extent of I guess fate can’t be changed. Meaning that trying to keep SM and TK away from each other wasn’t working.
My theory is that Sailor Venus and Artemis knew of Mamoru’s arranged marriage all along and maybe even had a background hand in the arrangement. The plan could be for Venus to pose as the Princess until Mamoru is married. After that, the real Princess could be revealed and it would be too late for her and the Prince to get together.
I think Minako is stressed out from continuing this ruse and from worry that the plan will fail.
For what it’s worth, that’s my theory.
Sincerely,
Luna Hope
(Aack! Evil typos!)