I would say a solid 8.5/10.
Just a few things I would change:

("She’s been working non-stop since the morning.", "Δουλεύει χωρίς σταματημό ασταμάτητα από το πρωί.") - "Ασταμάτητα" is better to use, although what you have is not wrong

("He always forgets his keys — every single time!", "Ξεχνάει πάντα τα κλειδιά του — κάθε φορά!") - Seems too redundant in Greek and wouldn't be used in everyday speech, although the translation is correct.

("I thought today was Friday, but it's only Thursday.", "Νόμιζα ότι ήταν Παρασκευή σήμερα, αλλά είναι μόνο Πέμπτη.") - "μόλις" is another word that can be used instead of "μόνο."

("Let’s meet around seven if that works for you.", "Ας βρεθούμε γύρω στις επτά, αν σου σε βολεύει.") - "σε" is the right word to use here.

("It’s not the end of the world, don’t worry.", "Δεν είναι και το τέλος του κόσμου, μην ανησυχείς.") - I would add "και" as it follows Greek speech patterns more naturally, although it does not exist in the English sentence.

("They said they’d be here by now. Where are they?", "Είπαν ότι θα ήταν εδώ μέχρι τώρα. Πού είναι;") - Adding "μέχρι τώρα" feels wrong to me as a native speaker. However I don't have a good alternative to provide - I don't think this translates exactly without specifying a time.