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It sounds as if you have done a good job of troubleshooting the situation.
Given that the problem is common to the whole house rather than just one point of demand isolates the problem to the supply tank or the tempering valve rather than an individual faucett issue.
You stated that the discharge line from the water heater to the tempering valve is hot, so the obvious point of failure is the tempering valve.
To prove that you might run the water till the problem occurs, then check the water heater discahrge line to insure it is still hot, If so, we can be confident the problem is the tempering valve.
The solution would be to turn off the water pressure and dissassemble the tempering valve and check for sediments or other problems that are effecting the valves performance.
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