1. Philippa is part of the Lodge, which is essential to the main plot.
I wouldn't call them essential. There is no real reason why this plot point was written in that way in the first place.
Why are exactly 5 mages needed? They could have written that the 3 mages, who are there anyway, are enough. I'm sure Avallac'h is more powerful than any of the lodge sorceress and he knows Aen Elle magic.
They could have let Moussesack's druids take the part of the lodge sorceresses
They could have have used Emhyr's mages to help Triss and Yen.
They could have just written that Eredin will be too proud too flee in the first place and there is no need for magic
The whole "we need lodge sorceresses", which we see for 10 seconds was just an excuse to include the rescue mission in Nobivrad and the Emhyr's boat mission in Skellige. Though I don't think any of them had any purpose or added anything to the game. Quite the opposite, it hurted the 3rd act imo.
The whole section after Kaer Morhen should have been dedicated to three things:
a) Aen Elle, including the Wild Hunt
b) White Frost
c) Relationship between Geralt and Ciri
I'm convinced if they would have cut out every mission that didn't have anything to do with these three points and replaced them with content that adds to them (like the cut miussion "In search for Caranthir", a look on the relationship between Avallac'h and his "creation" Caranthir would have been thousand times more interesting than breaking into a prison to save Margarita for a 10seconds cameo=) the third act wouldn't have been such a mess.
Though I agree that replacing any character just for "more romance" time is ridiculous. No matter how unimportant the character might be for the plot.