Translation:My friends and I could not decide.
Duolingo's translation is correct. "uns" doesn't add emphasis here. It's part of the verb "sich entscheiden".
Thanks. Since I've seen entsheiden elsewhere here used without the reflexive sich-form (and, if i recall rightly, without an object), would there exist a nuance in this case putting the emphasis more, as in the translation in the link you provided, on making up one's mind, whilst entsheiden by itself drawing attention rather to the decision itself?
I think this link explains it thoroughly :)
Konnten is the past tense of können while könnten is the subjunctive mood for können.
This page is helpful for understanding the subjunctive:
'könnten' describes a theoretical possibility (They could not decide if ...). 'konnten' describes a real possibility, so it really happend. The sentence 'Meine Freunde könnten sich entscheiden' is implying that they really could NOT decide, because of the weather or whatever. The sentence could continue like "... aber es regnete" [but it rained] or "falls sie zuhören" [if they listen]. The sentence 'Meine Freunde konnten sich entscheiden' means that they really decided. I hope this explanation helped.