You could also just stick to translating es = "it", das = "that".

Yes it is correct but not the right translation. If you would ask "Is that his dog?" it means "Ist das sein Hund?" in German not "Ist es sein Hund?".

Because "sein" only means he posessive (his). If you wanted to say they, the possessive of "sie" is "ihr/ihre". A video that helped me is here: https://youtu.be/CY4UhFttoog

it should be i think. That brought me here

It's not English. The sentence makes no sense. There are a few things wrong with that sentence. But the main problem is there needs to ve an apostrophe. it's dog.

It's grammatically wrong... not English

I think it's not, because the verb "sein" doesn't work with the accusative form. I'm not sure if this is the right explanation, but when you are using the verb "sein", the "Hund" (dog) in this case is the subject, and not the object.