ESTROGEN AND A MANGO LOCO MONSTER

(A screenshot from the Palais Garnier's seat listing stating the best seats for viewing and the view from the box five via this video)

Having been there myself in late May, I found an answer to my own question and I'm gonna share it with you guys because maybe someone else was asking the same thing!

Although yes, the stage is half cut off, it's one of, if not the, best seats acoustic wise. You're a perfect distance from the orchestra as well as the stage for everything to sound just right. As much as The Phantom would've loved the operatic performance, I don't doubt he would've been more focused on the music itself as well as the vocals, and, mainly, Christine.

Further, although going to the opera was more of a social thing than an entertainment thing, so the boxes were built for aristocracy to be seen above all things, you can disappear from public view quite easily in that box. There are two to three rows of seats going backwards to the door, so all one would have to do to disappear from sight of anyone on stage or in the audience would be to just move a seat backwards (which means he wouldn't have been able to see the stage at all, but would still be able to hear everything perfectly well).

Plus, the box is located right at the end of the row of private boxes, as well as very close to entry and exit stairs, both public ones and private ones meant for stage hands and general workers.