Of course this is all really just a thought experiment - nobody should expect to actually bear any tangible share of "manufacturing environmental impact" other than as a way to think about how "green" they're being when they buy and use any car, EV or not.
I suppose ideally the manufacturer would pay something akin to a carbon tax to offset the various aspects of environmental costs (similar to the idea of paying a recycling levy when buying tires, for example) and pass the cost along to the consumer. That would make the "manufacturing environmental impact" a tangible cost that the market would then factor into their decisions.
It's a little more than an experiment for me, because I am interested figuring out a reasonably accurate measure of the net CO2 from our household. My solar gives me a starting point about how many tons of CO2 I'm offsetting (about 3 metric tons/year), and the trees/shrubs we planted (about 0.5 metric tons/year), then then add the amount of CO2 from the usual suspects (diesel, gasoline, nat gas) based on consumption. Based on mileage/year from our ICEV's we're at around 4 metric tons, and heating our house added another 2.7 metric tons.
Back of the napkin calculation results in my contribution to the planet's death-by-CO2 at about 4.2 metric tons of CO2 per year. Of course, that doesn't add granular stuff like the cost of transporting the food and gadgets we buy, or our air travel - or my share of the initial CO2 created from the manufacturing of my car(s).
It's pretty eye-opening when you go run through the exercise of actually adding up your household's CO2. Unless I heat my house from wood (sustainably), and become grid-neutral using solar, I think it's nearly impossible to achieve a net-zero position on CO2. I have 41 solar panels on my roof, and don't have space for another 50 of them, and I can't afford building a solar farm. Without playing the purchase-carbon-credits shell game, net-zero isn't realistic.
I'm not at all in favor of carbon taxes, mostly because I believe that the money collected isn't actually being spent on reducing CO2. In Alberta, it's simply another tax we can't afford (especially now), and a transfer of wealth. The environmental programs being funded by the little of what's left over are dubious at best. As far as carbon credits are concerned, I've always been skeptical about how writing a cheque lowers my (actual) CO2. I think all that does is feed into the red tape (and possible corruption) in the name of saving the planet.