Svar is conjugated in Neuter (Svaret), but when plural it gets the "ne" tacked on (Svarene).
Similar to how The apple (æblet) becomes The apples (æblerne) and loses it's Neuter conjugation for a plural one. Which means we just have to remember that it's Neuter... I'm pretty sure.
It is correct. When a word ends with 'et', it is usually pronounced with so-called blødt D. It literally translates as soft D and if you don't know how to pronounce it, you should say L with your tongue behind your upper teeth. It is quite difficult for foreigners to distinguish from L sound. The other pronunciation is also correct... although it is only used when reading a book aloud or so. If you move to the country you'll mostly hear people make blødt D sound there, but if you say it with hard T, people will still understand you :)
It is because 'svaret' could be understood both as a noun in definite form (the answer) or a verb in form similar to English perfect form (answered). The second form is called kort tillægsform and is used in førtid tenses (førnutid and førdatid) together with verb at have or at være added in front. You will learn about those tenses further down in the duolingo danish tree, if I remember correctly :)