Can this be made without espresso powder, or is there a good substitution?

If you'd like to have the coffee flavor and just don't have espresso powder, you could sub some instant coffee.

Absolutely! Just omit the espresso powder. You can add a little more chocolate if you like. I simply like the flavor of coffee with my chocolate, but if you don't - just leave it out.

Either omit, or...

I might add a little Grade B maple syrup or dark brown sugar (just for that extra dimension of richness). Chocolate and maple syrup/molasses work really well together.

It's only a small amount, so you could just skip it. It does add a nice depth of flavor, but not a huge deal.

Yes; the reason you add espresso powder is because the coffee makes the chocolate taste richer and more complex. You can skip it if you don't have it.