Fred,
I apologize for getting in on this thread so late; I've been working on a troublesome old NASCAR Torino--yup, my heart is in Armor, but I like to get my hands dirty "in the garage" sometimes!
Ironically, I think that perhaps my Auto modeling may provide some insight into your problem? Specifically, I"m wondering if the use of enamel as a base coat is causing issues with your Tamiya overcoat?
In the world of Auto modeling, the trend has been away from enamels (in favor of quick-drying lacquers) because of their long "gassing out" curing time. In fact, the general rule for a great finish is to wait several weeks before even trying to polish out an enamel finish.
Enamels release gas as they cure. You can generally tell if they're close to "finished" by their smell. The gas has an odor, and I'm wondering if it may be the leaking-out of this gas which is causing an issue with the curing or durability of your top Tamiya coat?
Tamiya is not actually a mere acrylic; it's actually an acrylic-lacquer hybrid, which is why it's so tough. That's why I don't use a sealer in most cases; you don't need it. It's pretty durable. Another thing that may be an issue though is the fact that you may be getting adhesion problems over the enamels if the enamels have a smoother, semi-glossy sheen? Tamiya paint will not adhere all that well to a glossy finish if you do a lot of abusive weathering techniques over them.
Those are good oils you are using, but I'm not sure about your thinner. I use regular garden-variety "Low Odor Mineral Spirits". You MUST get the "low odor" kind, as it's distilled more so than regular mineral spirits and therefore weaker in caustic property.. I know that turpentine is a pretty caustic solvent, but I don't know about turpenoid? You said you're using Turpenoid "Natural"--that may be a stronger form of the stuff, i.e., not as distilled, and therefore stronger? I don't know about the "Blue" stuff either, but if it works for you, you win!
When I do the dot weathering thang, I apply the "dots" with a toothpick or in SMALL dots ONLY. If you apply too much "dot", you will definitely make a mess. Make sure you clean your brush often, between passes. I use copious amounts of thinner, and always dab off excess with a soft facial tissue. You want to leave a barely-perceptible sheen of colors on the model; it's really , really subtle.
MIG's weathering thiners and filters are enamel-based. You will notice that they take a lot longer to dry than a standard wash, and can be manipulated for hours afterward. I use them with the low-odor spirits usually, but have dabbled with their thinners as well. But as I said; they take longer to dry.
By the way, I don't like to weather over gloss coats as some guys do. I think that it causes the wash to slide off too easily; I like to have it "stick" to the flat finish a little more, for control's sake. I HAVE used some Future on my Torino decals and windows, however, in case anyone's listening! ha ha!
HERE is a solvent comparison chart. You can see that Low Odor Mineral Spirits have a lower "KB Value" (solvent power) than the others.
Hope this has helped!