I find it very hard to imagine an axle breaking from this kind of side loading. I can imagine that it would cause accelerated bearing wear and, if not corrected, eventual spindle failure. Bracing of the tongue itself shouldn't have any impact on towability (although it will make the trailer sturdier) - The center of mass, center of drag, and position of the axle should be the predominant factors.

I didn't think about buffeting caused by a large RV when I wrote my first post. You are not going to solve that problem by adding a large weight like a scooter to the front of the trailer, though. To resist buffeting, what you want is as much weight on the trailer's wheels with as long a lever arm between the ball and axle as possible (if you don't belive this, think about what would happen if you had no load on the wheels). Because the friction force is not quite linear with the load on the wheels, the lever arm term is slightly stronger, so it does pay off to move the axle back even though that will decrease the load on the wheels. Increasing the tongue weight in and of itself does nothing though.

I have been thinking about building a new trailer for a while, and I've been checking out other people's trailers at regattas. None of the ones I've looked at had more than 50 lbs. tongue weight loaded, and none of the owners complained about towability. Take it with a grain of salt, your mileage may vary, etc, etc.

Being able to manhandle the tongue easily is a very nice feature in a trailer.