What you're describing very often corresponds to this problem: a vacuum leak that disappears when the engine is warm. A small air leak causes a lean mixture when the engine is cold; when everything heats up, the materials expand and the leak seals itself, which explains the subsequent perfect...
He may have misinterpreted you when you walked around him (even if you did it cleanly). Some people take this as a personal "offense," especially if they're already tense or distracted
That's awesome to hear! It’s great that you were able to upgrade your 2015-2017 Mustang GT with the 2018-2023 MBRP Race Mustang GT catback exhaust and have it fit without major issues. The quad tip conversion definitely gives the car an updated, aggressive look that many owners prefer.
Checking for compatibility, Raceseng's reverse lock probably uses a "push" system or similar mechanism, which might be compatible with other brands' shift knobs. If you don't have luck with Raceseng, you might want to try shift knobs from other quality brands that use a similar reverse lock...