1: Prohibited, IIRC. I think it was because it disrupts the airflow over the horizontal. Mayber a current 208 pilot can answer; it's been several years since I've flown one.
2: Mechanically, not a problem either. From how I was told, having it in feather or Beta for longer times isn't wise because of the heat from the exhaust aimed right on the belly pod...if yours has a pod.
Again, not 100% on the answers, just what I was told/remember.
Caravanman, is correct the use of Beta in flight is prohibited (don't really know why you would want to anyway). When you pull the power back to idle and the prop is full foward, you now have an eight foot in diameter speed brake out there.
We always start with prop full foward. Just as Caravanman said, it helps keep air flowing over the pod (especially if it is booted).
I've done it in another airplane. The power lever was in a totally different configuration. Lemme guess, you're a jump pilot?? When I did it it was just during the initial descent from a high altitude and in ground effect like the feeder pilot mentioned while hauling ass on final and planting the landing right where you want. I would be very careful if you try it.
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.
Starting the prop in Feather is only an issue if you leave it there for an extended period. They say it can do damage melting/heating the cargo pod if you have one. It has been told by several authorities to start it out of feather for this reason. I would love to hear anyone elses input on the issue.