Why "domum" and not "domo"? I don't understand when we use each of them.
Okay thank you for restoring some of my brain cells that have vanished while trying to comprehend these weird Latin things
Are there any other forms of "dom-something" or any other nouns that do this?
What is wrong about the following sentence: Stephanus et Livia domum it? Would it have to be pluralized?
Yes, it would have to be pluralised. The subject of the sentence is two people, therefore the third person plural of the verb, i.e. eunt, is required.
The Romans sometimes employed a singular verb for a neuter plural subject, probably through Greek influence, so sometimes you can have the sort of situation you are asking about. But normally there must be subject-verb agreement.
It is plural