Discharge efficiency varies from battery to battery. Generally speaking, it's a 90+% efficient process (as is charging). However, there are certainly exceptions which are more or less efficient.

Transmission costs probably eat a bigger portion of EV total efficiency. Those depend on how far you are from your power source and what voltage your power is transmitted at.

That said, a thing EVs do that ICEs can't is accept power from any source. That means that only a fraction of the fuel is actually generated from a fossil fuel.

One other thing to consider is that as power density goes up for batteries, the amount of required batteries go down, ultimately reducing vehicle weight and improving range.

Assuming batteries were ever 10x their current power density, you'd probably not see cars with 10x range, rather, you'd see something like 400mile range cars with the same total power capacity of yesteryear cars. Pretty much all because you've cut the pack weight down by 1/10th (probably less because you need a lot less infrastructure around maintaining temperature for 1/10th the number of batteries).

The inefficiency inclusion from the generator for electric is somewhat unrelated to an ICE. No ones including the inefficiency of extracting and refining the oil, the huge amounts of time/energy required within the earths crust to create the oil, nor the energy conversion from the sun into organic matter in the first place. It seems a lot of the details on the fossil fuel side get left out.

I wouldn't say you would really count battery discharge efficiency, usually a batteries energy density will be how much useful energy you get out on discharge so it already accounts for charge/discharge efficiency.

Same with electrical generation with generators or whatever. One of the best things about BEV even if run off fossil fuels is that power plants don't care to much about space or weight, they can do multiple levels of heat recovery and whatever emissions controls and get higher thermal efficiency with lower emissions and all that complicated gear is not being carried around in your car nor maintained by you. On top of that now you are decoupled from energy source so changing fuel oil to natural gas or mixing solar, wind or nuclear doesn't change your vehicle.

Also the ability to recover kinetic energy during braking which is lost with ICE engines.

Most of the energy of combustion is wasted through heat loss. One way to increase efficiency would be a secondary system that harnesses the heat output.