Good work, that's a much better wrap of the belt. The TPS wouldn't cause the current rich mixture, even if it was off when tuned. The PCM tests for the TPS signal at each start, and sets idle baseline at the new value each time, zeroes it at that value each time. What matters is a smooth linear TPS signal. How large is the fuel return line, and is the regulator capable of flowing the amount of flow going through it? The regulator is supposed to vary the pressure based on the vacuum/boost it sees. It sounds like it's not able to divert enough under boost, or increasing the pressure too much.