I've been active in the miHoYo leak subreddits daily for well over four years at this point and have been involved in arguments substantially more inane than this so like, as long as it's not gender wars it's no biggie for me.
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The Tingyun part felt tacked-on yes. I think this storyline was meant to be just setup for whatever they're planning in the future for 3.x and that's why they felt the need to introduce her and have her as a player in the story from the outset. This isn't meant to be like a triumphant conclusion to her arc but its beginning. By making it an interlude you make it an obligatory(-ish) part of the storyline which a character quest couldn't have accomplished. I still feel like she should've appeared in the main story and then had a CQ afterwards to flesh some stuff out a bit.
Believe it or not I actually kind of like the choice to have had our meeting with her be offscreen and have them be like "yeah we had her the whole time", I think it's a fun reveal, but I'm also not a big fan of how she acts like she's been our pal for years.