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Is it possible to go from TTT firm + TTT school/subpar grades + prestigious inhouse to biglaw? Originally biglaw rejected me on the spot.

If you've secured a prestigious in-house position, I'm not sure why you'd want to go elsewhere other than lateraling for in-house promotions. That being said, you might be able to get a counsel/non-equity partner somewhere. It'd probably be slightly better pay for significantly worse hours and a very slim chance at making equity partner, not because of the status of your background but just because you haven't been developing the skills/experience in biglaw that would make you an attractive fit and you're not coming over from a prestigious gov't agency/post that often translates into partner (e.g. AUSO/DOJ/Etc. leadership).