Translation:Cake is delicious.
I put "the cake is tasty" and "cake is tasty" and got it wrong both times. nothing was underlined, but it seems weird that 'tasty' suddenly stopped working as an acceptable translation. I reported it for "my answer should be accepted" just in case
I believe " 맛있어요" is more accurately translated as saying something has flavour or taste -(맛=flavour/taste 있어요=it has)- rather than saying it is "delicious", so if anything your answer should be closer to correct!
Would 케이크가 맛있어요 also be plausible?
Not really, because your sentence makes it seem like you are talking about 1 cake. The sentence they are giving us is talking about cake overall.
I think its because it's more polite and more effective with this word usage
If 케이크 were the subject of a sentence, would it be 케이크이 or 케이크가? It seems more fluid to say 케이크이, but 으 is technically a vowel, right?
Yes, 으 is a vowel, so 케이크가 would be used over 케이크이. However, keep in mind that 가 and 는 are used in different ways; although you could say " 케이크가" it would imply that you were talking about a specific cake rather than cake in general -aside from a few other little nuances- so you would switch between the different particles depending on the context.