When forward of the ground idle stop, and when in the alpha range or forward of beta, you have control over the propeller, and the propeller has safety constraints. When you move into beta, you take those away. If you do so at a high speed, the prop can quickly overspeed, as it's being driven by the slipstream and not the engine, and your drag may increase significantly without control. You may lose control over the propeller.
Descend at flight idle, and come into the ground range on the ground.
There is no harm in starting the propeler in feather, as the engine isn't mechanically driving the propeller. It can be held still the engine started...not an issue. Whether you elect to move the propeller lever forward prior to the start is up to you. Where I leave it depends on the aircraft and the configuration with the PT6, but you're not going to help or harm the engine either way, generally speaking. If anything, feather saves the circulation of oil during the start process for the engine alone, but the feather isn't goin to drive out of feather until adequate pressure is available and rotational inertia has overcome propeller resistance, anyway.
Extended operation in feather isn't a good idea as you're reducing or robbing airflow through the oil cooler, and you can see an oil temp increase with a potential pressure decrease that's unnecessary.