As you have shown ,Things I've picked up reading this thread and exploring a few rabbit holes along the way.
No US or Canadian distributor has chosen to import Continental tubes. I'm sure it's a business decision, but not sure what's behind it. Perhaps better margin on other brands, perhaps as a low cost leader, there is more net sales opportunity is selling a slightly higher priced tube at the same margin at scale.
There is a distinction between radial tubes and bias tubes, but talking about them that way is misleading. There's not really such a thing as a radial tube, it's more correct to think of them as tubes made for radial tires. Many motocross and off road tires are still bias ply tires, don't build the same amount of heat during use. Because they can be susceptible to pinch flats, many riders prefer to run heavy duty (thicker) tubes. Those tubes are not appropriate for use in road applications because they build and retain more heat than thinner tubes. The tubes used in radial tires tend to be thinner, more flexible, and retain less heat when in use. The Continental tubes are spec'd at 1.5mm, and the Metzler's between 1.7mm &1.8mm. The tubes for bias ply tires can be up to 4mm thick.
There is a significant difference in the rim between tubeless and tube type where the valve hole is. On tubeless type rims, there is a flat machined around the valve hole so the value can be set flat and seal properly. On a tube type wheel, the value seat integrated into the tube does the work of preventing tire from leaking from the value. As such, most tube type rims have no special treatment around the valve hole.
Speculation: I'm guessing that the reason it was recommended never to run tubes in tubeless rims with tubeless tires has to do with the valve hold situation and with the internal rim profile. It may be that the machined edges of the value stem hole are sharp enough to cut tubes. It's also possible that the valve stem seat on an inner tube won't set properly in a rim that has a narrow channel where the value hole is drilled. Again, just speculation. If you were running an inner tube on a tubeless rim and got a puncture in the tube near the valve stem (tubes do move when in use, and there is expansion and contraction that happens), the tubeless tire would loose air because the valve is not sealed to the rim in the same way it is when running a true tubeless setup.
Image of the inner rim profile on a 2014 r nineT. These are not traditional tube type rims. They do have the bump designed to hold the bead in place after the tire is mounted, which means that in theory, sealing the rim and using a tubeless tire is less risky than trying to do it on a rim that doesn't have the bump. My local shop was nice enough to snap these for me while replacing my tube and remounting my tire yesterday.
View attachment 161119 View attachment 161120
we have bead lock tubeless spoked wheels,
my 2018 has the same with a slightly different rim well design,
you will see MT and rim width in the markings,
it has a shallow taper to grip the inflated tyre , the raised inner rim edge prevents the tubeless tyre bead moving inwards holding it in place and air tight at the rim,
some early ktm and possibly others had morse taper, MT marked rims without the bead lock,
many tubed rims were marked WM, no taper or bead lock,
the mechanical motor-cross bead lock/clamps are a different set up for off road use,
we all have different preferences on our bikes, including tyres,
my personal choice is tyre secure on the rim, and tubeless tyres,
Road speed rarely gets to 100 mph,
the quicker turn in with less effort and suspension compliance with tubes removed works for me ,
the later shallow well cast and spoked bmw rims do need a bit more technique to dismount/mount which is reassuring when the flat tyre wallow starts,