The issue with the heater control solder joints, is due to the way the parts are installed. Had a similar issue in a GM instrument cluster. Bad solder joint on a high wattage resistor. It comes down to the assembly factory (say in Mexico or wherever) not deoxidizing the part properly before they try to solder it in. As to all the other minor / major gripes, well you do know Mazda was bought by FORD ... so what do you expect of FORD Products ? ?
May be correct? It's history, not a "may be correct".
In fact, if not for Ford's investment in Mazda in 1979, Mazda might not exist in its current form. Ford, after all, essentially saved Mazda from becoming just another nameplate that existed in people's memories.
Dealer and other repair shop wants $3000+ to fix it for a $1500 part and $1500 labor.
A shop says they can do what you described as "cleaning" for $300. $300 later, problem not fixed and you go over and described in detail what needs to be done, as well as showing them exactly what to do, and $1000 later, issue you want them to fix is fixed.
Dude, this totally is on you. $300 would be way too cheap on the "cleaning" job and you knew it. How do I know? You said the cleaning will take 10 hours do it, any shop I know of charge $70 + / hr, and at $300, the job is way too cheap. You probably were too happy to ask why it is so cheap compare to the $1500 quoted labor for replacing the intake manifold.
And you actually saved $200 on labor, not to mentioned don't have to buy a new intake manifold.
Just to play devil's advocate here, but how can you know the entire upper + lower intake weren't completely gummed up from your cat melting, blowing half burnt exhaust sludge back into it and then you driving for what you say is a long time without being able to get it into higher rpms and blow it out?
How can you be sure the shop didn't clean the SSV correctly the first time? Are you positive the sludge you found afterwards wasn't already in your intake that you had already been told more than once to replace? As in a part you didn't pay them to clean blew into the SSV and dirtied it again.
You ignored what someone already told you needed to be done, you decided to diagnose the problem from your computer chair and you asked the shop to perform a very specific service completely ignoring what people had warned you before. You got exactly what you asked for and now you're badmouthing the shop online because you called it wrong?
To be honest and no offense, most shops try to avoid customers like you.
If you want to play armchair diagnostician, don't bitch at the shop when your diagnosis is dead wrong. If you wanna listen to what people in forums say, then you need to be getting your hands dirty verifying your car has the same problem and fixing it yourself.