Hmm…you should be able to lean the mixture with your hardware lever at any time. The cockpit lever will move to keep the core-sim mixture set at a level that corresponds to the mixture percentage on your hardware axis, but if you move your hardware lever, the code will react by setting the core sim mixture to match it. Any chance that you have automixture enabled in the sim settings?
The real-life turbo arrow has a fixed wastegate. It requires adjusting the throttle throughout the climb because the wastegate doesn’t move. Basically, it was a way for Piper to offer more power at a low price compared to the competition. Most turbocharged planes have a variable wastegate that keeps manifold pressure constant, like you described. The turbo arrow is quirky in that respect.
I’d be happy to continue discussing on the mod thread (linked below) so we can avoid cluttering this bug report thread with mod details.