Is it true that by having Magneto in your party in Mephisto's realm, you can save both Jean Grey and Nightcrawler? If so, whose "spirit" will Mephisto summon and what ending will play in the destiny epilogue?
Yes it's true. The spirit summon is random. The ending will just skip over that part
Honestly, it feels more like a bug really, characters still speak to you as if one of the X-Men died.