15A 125V smart plug being used to slow charge electric car
NEMA 6-15 duplex recep does not approve.

Nope.  Nope.

Take that NEMA 5-15 to 6-20 adapter and throw it in the trash. What are you thinking?

Have you deliberately wired 240V to a common NEMA 5-15 socket? No, that's unbelievably dangerous.  Get the correct 240V socket (one such pictured above, they also make a near identical NEMA 6-20 20A version). 

Plug your charger into that, like it's made to. 

As far as smart-switching it... 

Kitbash this out of UL-listed stuff. It's not hard.

All you need to do is get a "RIB" Relay In Box type of product.  It's actually made for use in mains wiring, it has a UL listing instead of ЯU.  It mounts in a junction box knockout. They also make contactors that mount on a junction box cover. 



Select a 240V relay/contactor (honestly, I could care less if it's 2-pole) that has a 120V relay coil.  In the junction box where you mount it, come out of that with conduit to right next to an actual 120V receptacle.  There, you fit a common 120V NEMA 5-15 inlet.  Get it? 

The wiring is simple: from the 120V inlet hot/neutral to the relay coil.  

Now, plug an extension cord from the inlet to the outlet that's right there.  The relay picks up and powers up the EVSE. Getting the picture? 

Now, where the extension cord's plug meets the ordinary NEMA 5-15 outlet, you put your smart switch there. 

Another option is to use the very common/cheap 24V contactors that mount in knockouts or box lids. They have strict separation of 240V from 24V.  For that, still use your smart switch, but plug a 24V transformer into the smart switch output. Basically the same deal, except you get to use cheap thermostat wire and informal wiring methods because it's low voltage and thus mostly harmless.