I'm not sure I can offer any constructive advice different than what has already been written.
I own this lens. It pretty much matched my 100-400 L in sharpness. I did end up buying the 70-300 L (ridiculously expensive, but I just couldn't resist and frankly it probably is worth it) Obviously, the 70-300 L is sharper, faster and better built, but it is also nearly $800-$900 more expensive.
I did have some problems with the Tamron occasionally hunting and hunting unable to focus, but that seemed to be under very limited circumstances and shutting the camera off or manually focusing the lens to get it in the general range of where I wanted to focus seemed to fix that. I have never had any problems with the OS/IS system, although it does make a bit of noise and seems to rattle a bit if you shake the lens.
I know this isn't exactly helpful, but it seems like you are throwing good money after bad. Like others here, I would say bite the bullet and send it straight to Tamron, explaining the situation and the repair history. Maybe they will take pity on you and repair it at no charge.
One problem with a retailer's repair service is that they may not get enough of a specific lens in for repairs to really know what the problems are. They may be very good at routine repairs, but when you have a persistent and complex problem they may simply not have the experience and expertise to properly diagnose and repair.
I understand your frustration, but if the retailer won't make it right (and it seems you have tried numerous times to get them to do so), I would take it up with the manufacturer and then find another retailer in the future.