Right now I think the carburetor is probably the problem. The replacement carburetors cost like $30 to $50 and an OEM carburetor is $170 to $250. It might be possible to get 2 bad carbs in a row.....
Let me know your thoughts. I could really use the help
The last warehouse I was involved in running rented their spare pallet spaces to ebay/amazon vendors ,
Nothing odd to despatch the same items with a dozen different letterhead invoices from the cheapest with plain B & W A5 invoice to the most expensive with full colour A4 invoice with lots of photos of the supposedly factory on it.
Items were all the same , some times we stuck different brand name decals on them, particularly with the stuff that came in plain white boxes.
So yes very possible to be sold 2 or even 10 defective carbs from the same batch from what to you appears to be 12 different vendors who are all the same sleezebag ,
And there is a big difference between certified OEM parts with a warranty and a defective part that the OEM factory sold originally as scrap metal.
Both can be legally sold as OEM
Also a big difference between a defective part that "FITS" a particular machine and a good part that will WORK on the same machine .
You seem to have done a good job of diagnostic testing.
As it runs fine on on either cylinder alone but backfires through the carb with both sides firing that eliminates the magneto .
So try adding a bit of choke very slowly
If the backfiring initially gets better then gets worse it would indicate a blocked carb fuel passage.
Next pinch off the fuel line with something like a clamp so you can do it slowly
If the backfiring goes away or at least gets better then that would indicate flooding or too high a float level .
To get a backfire in the muffler you have to be passing unburned fuel into the muffler that either ignites on some thing in the muffler that is red hot or is ignited by the following exhaust pulse
You can also try opening up the exhaust valve lash to around 0.010" which will delay the following exhaust pulse just a tiny amount .
You might also have a restriction in the muffler