gradually the heat died right off to nothing,blowing cold air only, replaced thermastat and flushed cooling system, the control **** for the amount of heat feels pretty flimsy.

 

One thing it (may be) is that the Heater Core is blocked.
A blocked heater core won't cause the engine to over heat, but will cause no exterior heat.
Run the engineuntil itheats upto normal condition, then with your hand, feel the hoses going into the firewall(under the hood) that connects to the Heater Core....they both should be as hot a the raditor hoses. If one of them is not hot (which would be the heater core outlet hose) then the Heater Core has a blockage. If this is the case, sometimes, if you "Back" flush the cooling system, it will unclogg the blockage; if that doesn't do it...then the heater has to be replaced.
If the heater core checks out to be fine, then you may have a bad temperture control unit (in dash) as you mentioned.

 

If you have a cable for the heat control - it could have broken also. You may also want to check if the fan is running. If the fan is running and you've checked the heater core it's possible you could have broken the cable.