Re: Boat position on trailer
If your trailer is reasonably adjustable, you should be able to move the boat to get the transom over the bunks, move the winch mount to accommodate, and move the axle to produce the correct tongue weight. If you need to compromise, could you extend the bunks to provide the support? This wouldn't fix weight distribution, but it would fix support of transom.
A 10" extension would be unusual, but doable depending on weight, construction of bunks, and construction of trailer. Extending the trailer beneath wouldn't hurt either, but at some point it would be economically better to trade trailers.
As mentioned above, pics would help.
Chip