This is one of the trashiest adventure I have ever played or run, but the plot is amazing. However, you should consider not allowing the celestial in this adventure in the party to help.
When I ran the adventure, nothing ridiculous has happened. When I played it, on the other hand... BBEG has failed a saving throw against this ally's plane shift spell and was sent to Mechanus... Oops, great fight ended in just one failed saving throw because BBEG does not have Legendary Resistance.
[5 of 5 Stars!]