Re: tongue weight
If you ever towed a large travel trailer you will know that the "long lever" you mentioned is a disadvantage when being passed by a semi. The air wave off the front of the truck hits the back of the trailer first tending to push it to the right. The tongue therefore moves left and hence takes the tail end of the tow vehicle with it which points the nose of the tow vehicle toward the ditch.
You need to compare the lever arm in front of the axle to the lever arm behind the axle. That's exactly the problem in trailers that are shaped like a giant brick. At 10% tongue weight, there is 3/5 of the box in front of the axle and 2/5 behind. That lever arm behind the axle gives the back of the trailer a lot of control over where the whole thing goes as you note.
I note that you use the example of a travel trailer, not a boat. Because boats are just plain far more stable than any travel trailer.
Compare that to a I/O or outboard boat. At 10% tongue weight, the axle is closer to 9/10 of the way back. Nothing is back there catching air to make it unstable. Rear engine power boats simply don't need 10% to be stable.
Tongue weight is tongue weight. 5% is too little
Then how do you explain open top aluminum boats with outboards? 30% to 40% of the weight of the boat is in the engine, behind the transom. They typically have the axle within a foot of the transom, practically at the rear end of the trailer. Even then, the tongue weight is under 5%, often around 3 or 4%. And they are dang stable, narry a wiggle when passed by a semi - even with a 20' boat. No worry about the tongue going negative weight, because unless you hang something from the engine, you can't shift the weight back.
I have seen a guy go down the hill in his boat at his house when he did just that. No damage but a heck of a ride and fortunately no cars or kids were in the way. Another reason to use wheel chocks.
Regardless of tongue weight, chocks are needed. The tongue jack wasn't designed as a parking brake. Those that don't chock will eventually find a steep enough hill that the trailer will run away on it's own.