Using these amazon adapters with stock prime charger?

i dont think that works althought the 6-50 shape seems to look the same as 5-15 plug.
The 5-15 plug is smaller that will not fit into the 6-50
Here is the nema chat

 

The short answer is NO; it doesn't matter that its UL certified - because your not using it for it's intended purpose. So if you burn down your house - good luck trying to collect from your insurance company or trying to sue the company that created this plug.

As far as those "permanent adapters"; people are making - they are not permanent. They need to be taken apart every other year, inspected, everything tightened, connections tension checked. Pushing 16A through that adapter, day in and day out will loosen those screw and clip connectors over time. At a minimum; you should hit the connections with an IR temperature gun every 6 months; while its charging the car. Loose connections will heat-up; before failing, arcing, or sparking.

Hope this helps...