Translation:My grandmother and I play the piano together.
No, I'm not implying that at all. If you read my next comment (reply to Xanderificus) you'll see I agree that reasons for the speaker's gender differing from first person content gender could be construed contextually.
My point is simply that as DL has both gender voices available, they should align genders where applicable. Out of context, the most natural assumption is that the DL narrator is just someone talking, and in this case first person content should match gender with the speaker.
If you're translating from the English to Spanish and the gender of any of the subjects is masculine, unknown, or, in the case of DL's first person audible questions, irrelevant, then "juntos" is fine.
But this question asks us to translate from the Spanish, so I'm unsure how you were required to use "juntos", unless it was a 'write what you hear' question, in which case "juntos" would be wrong (although, as mentioned above, it would be natural to expect "juntos" if the sentence is spoken by a male voice).
"Marido" and "esposo" are synonymous and interchangeable. It's possible their usage may vary geographically, but basically there is no difference in meaning.
One difference that does exist, however, is that while "esposo" has its female version "esposa", there is no "marida" for wife. Instead it's "marido y mujer" for "husband and wife".
Another difference is the verbs each is associated with. When referring to marriage you would normally use "maridar" because "esposar" means to handcuff :)
Re "jugar" and "tocar", the former refers to playing sports or games, the latter playing instruments or music. Spanish speakers must find it odd that we use "play" for both.
It's technically wrong. The correct use of the ampersand is in terms that denote a single entity or thing: Johnson & Johnson; Rhythm & Blues etc. It technically shouldn't be used to replace the conjunction "and" in any other circumstances.
That said, it was once acceptable, and given how commonly it is (mis)used today, it's possible that in the future it will become acceptable again.
Excluding the word "the" changes the structure a little: "Piano" becomes a verb modifying noun instead of a direct object. This wouldn't be a problem, unless the sentence referred to playing a specific piano instead of playing the piano in general. In that case it would be necessary to translate including the "the".
Out of context it is highly likely most people would interpret the sentence as meaning playing [the] piano in general, but textually the other possibility exists, so including the "the" is a better translation.
I put juntos and was marked wrong. Junior's sex is not determined here. Duolingo has always stated that when a "duo" of mixed sexes is is being described - parents, grandparents etc... - the masculine version of that word is to be used. So why have I been marked as incorrect?
Definite article (not pronoun) exclusion / inclusion before object nouns is confusing in Spanish, but it's no worse than English. It's just something both languages do, often seemingly arbitrarily.
If you ask Spanish speakers why, they generally reply "Así es como es", and English speakers do the same, "That's just how it is".
There are certain guidelines, but they're confusing in themselves. The best advice is just to immerse yourself and get a feel for it.