"These girls are beautiful" is an accepted translation.

If it did not accept it for you, there may have been an error in Duolingo somewhere - a link to a screenshot would be helpful so that we can pass it on to developers.

In everyday speech, yes, although people sometimes add hier and dort / da to denote whether an object is close-by or further away (e.g. diese Blume hier, das Auto dort, die Häuser da). In written German, you may encounter jene/r/s / jene for that / those.

As a rule of thumb, I'd recommend using dies only for "this, these" (near things) -- and using only das for the neutral "this/that, these/those" (near or far).

Most of the time, yes.

Technically, these = diese und those = jene. But jene is not very common any more and sounds archaic, so most of the time you don't really differenciate between "these" and "those" in German.

Look at the word before it and/or at the verb.

Dieses Mädchen ist schön. “This girl is pretty.”

Diese Mädchen sind schön. “These girls are pretty.”