This is going to be A LOT of ...mostly broad questions. I'd recommend that you find the questions you are best qualified to answer
I've heard they used a sort of corporatism/national syndicalism, I've checked the Wikipedia articles on them but it hasn't helped.
-Lets say I'm a car mechanic. In modern America I work for a company that owns and runs these shops, I work and they pay me for that said work and don't have much a say on what goes on there unless there's a union or if I get promoted. I vote whenever elections come about. How is the economic and political side of things different for me being a mechanic in a fascist nation?
-Do models of national syndicalism or corporatism have any merit? Or can they be tossed into the waste-bin along with Hitler's anti-Semitism?
Were any sort of constitutional rights ever put in place (then just later ignored)? Were they ever proposed? Would it have been possible to legislate and uphold them? Or did even on pen and paper was the government allowed to be dicks?
I've heard Mussolini was dismissed by the grand council. What was the Grand Council? Did all fascist nations have an equivalent to this? What were their powers? Were they legislative, executive, or both? How would one become a councilor?
Were any of the fascist nations what we could call economic successes? Could they have been if they decided not to go to war or did certain things differently? Was social welfare raised with "cooperation between labor and industry" or was it just as coercive and unequal as unbridled capitalism?
Were all fascist, from its philosophical beginnings to its wartime actualization, such traditionalist/reactionaries? Were there a significant amount that weren't religious, racist, or upholding of "traditional values"?
-How similar are the mixed economies of 40s-60s America and modern western Europe to fascist economies?
I can only really answer a small portion of the third bit with some extra information about Germany during WWII, and it's not a lot, but you can draw several conclusions from it.
When reading about German armor production (both pre and post Speer) it was mentioned that Germany used the "batch" system of production. This, of course, meaning that instead of having assembly lines, they would simply create x amount of y and send it off in batches. It's mostly covered in regards to the Panther tank, but you can infer a few things from this system. First is that standardized parts throughout most of Germany during WWII just was not a thing, although they did have several vehicles on the same chassis, there was never really an effort to standardize things in armor production until the Entwicklung series of tanks that never came to fruition. Without standardized parts, repairs and supplies become haphazard because everything uses parts and supplies specific to that vehicle. Obviously I'm not a German mechanic from WWII, so I have no idea how deep this runs, but it was present enough that it caused problems.
Another thing, which might have no bearing on German industry what-so-ever, is that a large portion of vehicles that were put to the field were mechanically unreliable. So you now have a bunch of vehicles that are not standardized that are regularly requiring advanced repairs but require specific parts. I'm not an economist, but it's pretty simple to figure out that at that point more resources and assets are being put towards simply fixing and transporting these vehicles to the front, where they typically just broke down or were abandoned anyway. Awesome.
There's the myth (it might not be one, someone more qualified should comment) that during the 1930s, Germany had it's unemployment rate to under 1%, at which point, I would say that is an economic success...to a degree. Once the war starts, Germany begins to repurpose vehicles from other nations to work for them, they have poor supply lines, and shoddy equipment in several vehicles. All of the processes involved were state owned...and none of them moved to something like the assembly line where one person does one thing over and over again, and generally, gets really freaking good at it. So post-war starting, I would say that Germany was an economic failure because they failed to adapt to a (then) modern wartime economy that could actually supply it's armed forces properly. Combined with the fact that Germany never really produced food much beyond pre-WWI levels and the otherwise large amount of money being spent on military budgets, I would easily classify Nazi Germany as an economic failure.