Ian,
I suspect that the term "overspeeded" may be misleading in this case. I believe the intent of the term "overspeed" isn't mean to imply that the pilot in question actually caused a propeller overspeed, but rather brought the airplane inadvertantly back out of feather.
A typical propeller at that time was a hamilton standard hydromatic prop. This required a dedicated oil pump called a "feather pump," and this pump had to be energized to drive the propeller into feather. The pump was supposed to shutoff when the propeller was feathered. To drive it into the feather position, a button was pushed, and the button was held in by a solenoid. When the feather position was reached, the button was supposed to pop out by itself, interrupting the pump process, and leaving the propeller in feather.
A common problem with the hydromatic prop was that the button wouldn't pop out. The solenoid would hold the button in, and the propeller would feather, then with the pump running, continue to drive itself out of feather again. The solution was to guard against the button, watching the propeller, and popping the feather button out by hand once the prop had feathered.
An integral part of training in multi engine airplanes is performing engine-out work, just like it is in single-engine airplanes. As complexity grows, so does weight, and performance on reduced power or less engines tends to suffer, accordingly. Procedures tend to become lengthier and more exacting, and the requirements or demands on the pilot also tend to increase.
I don't know all the circumstances regarding the loss of your father, but it's important to realize that one can do everything right in an airplane, and still pay the ultimate price. In this case, the actions with the propeller don't appear to have been the cause of the mishap, but rather striking the tree. How the two events may be related, one can't say with such little information. However, when an emergency develops, one can only contribute toward improving the situation such as one is able; there are never any guarantees that one will be successful, and accordingly there is no ringing condemnation when one is not. One can only do ones best with what one has available.
I'm sorry to learn of your father's passing long ago, yet would urge you to remember that this incident does not speak ill of your father. It's quite conceivable that your father did the right things, and was the victim of a situation which simply deteriorated beyond his, or anyone else's control. Simply put, we can all fly into places from which we cannot return, and in my opinion, some of us are destined to do so...no matter how much we might wish to escape the fact. We must all go at some time, without regard to our particular skill, circumstance, age, caste, or form of employment.