Very simply, we are not making assumptions on what we've done, but rather what the OP is planning on doing.
This is a quote: "after doing some measuring we think it would only need the wheels for about 8 inches and then should be back onto the axles." To me that quote says that for about 8 inches the 5th wheel is going to be off the axles and supported by the caster wheels and the hitch. Now, does this sound like they "do not carry much of the weight of the trailer"? Or have you seen his driveway and can attest to the fact it will work?
Just because it's worked for you in some instances doesn't mean it will work for everyone in all instances.
I was offering an opinion, and stated as such. It's also my opinion that you could probably pull a 40' 5th wheel with a half-ton pickup, but I think that's a bad idea too. But I've never done it so I can't swear to it, just offering my opinion.