Your point is well taken on all issues. This is another odd sentence indeed and thanks for bringing ti to our attention. Now, if it were a really erudite baby, who can say? As for the others ...?
I really don't know what to do with this sentence. I'm assuming it means that the "baby has begun to talk". But we can't have such a long translation for such a terse sentence. How about "the baby has talked to the prime minister and the matter is settled.''
Your discoveries are a great help...keep them coming.
I can't really picture "the baby has talked" and you really want the present perfect. Ever thought of becoming a translator? I did technical translations Danish/English/German, which are very related languages. But sometimes you can't go word for word. For example, Germans use the present perfect as their general every day past tense in speech, while Danes use the simple past more often. And don't get me started on passive voice without indication of the "who-done-it" (old slang for mystery stories)!
"infant" is "βρέφος" occasionally we might call them "μωρό" but I doubt that they would be speaking. It's best to learn the natural use of the vocabulary.
P.S. There are times we might call an adult μώρο but that would only be for a very close relationship. Don't use it if you aren't sure of the usage.