Would 'he/she/it used the comb' also be an accurate translation here?
Yes. It's added now.
What is the difference between using the verb use in the form "Χρησιμοποιήστε" and "Χρησιμοποίησε"? is one of them used to give order to plural while the other to one person??
Yes, the form with -στε is plural or formal/polite.
In drop down help it has use and used are both correct
Yes; it can be either the command form ("use!") or the past tense for "he, she, it" ("he used" etc.).
According to the Duo notes, verbs that end in -ώ are supposed to be conjugated with an -α ending for the singular. Why is this not χρησιμοποίησα?
Χρησιμοποίησε is either the imperative (second person singular) or simple past (third person singular). The translation points to the first case.