Jeremiah,
Tongue weight in discussion is at a 'static' loading...that static load becomes much, much higher in a dynamic situation. Plus as you said...a single axle will have the weight tossed back and forth easier than multi axle trailers
Like in a whop-d-do, RR tracks, pot-hole, etc, etc A 400 lb static tongue might have a dynamic weight load hundreds or more
What it gets to is not easy to figure out. Even in a laboratory it would need tons is expensive and large equipment
Won't happen over night either. Those pictures are of failed spot welds that took a lot of time to get to the point where they gave up. The more over their design ratings...the faster/sooner they will fail
Why ask newbies how they drive, where they drive, etc that creates ever higher dynamic loads. Toss in compressing WD bars loading even more to exacerbate this whole loading situation during compression and the release of that compressed WD bar married with the TV Suspension release...
OP...glad you made a decision and posted back. Ask that either link this thread to your new purchase thread and/or post one here and then start your new thread
There are many, many lurkers who follow 'advice' here and can learn a lot from your thread...THANKS