それぞれ

Hello,

I have a few problems understanding if それぞれ is a noun or an adverb in sentences. For example, for these sentences from それぞれ【其れ其れ】の英語・英訳 - 和英辞書 - 英語辞書 - goo辞書:

少年たちはそれぞれ犬を飼っている (Each boy [Each of the boys] keeps a dog.／The boys each keep a dog.)
トムと太郎とフィリップはそれぞれ英語，日本語，フランス語を話した (Tom, Taro, and Philip spoke English, Japanese, and French respectively.)

In the above sentences, could you replace それぞれ with それぞれに or それぞれが?
And in case you can replace with both, is there any difference in meaning?

There is no difference in meaning among それぞれ, それぞれに and それぞれが in the fist example.
As for the second one, the meanings are ambiguous for all of それぞれ, それぞれに and それぞれが. It can mean "Each of them spoke all those three languages." Furthermore, especially when the subject is two persons (e.g. トムと太郎はそれぞれに英語、日本語を話した), それぞれに can mean "to each other (Tom talked to Tarō in English, and Tarō talked to Tom in Japanese)" since に indicates the target of 話す. You need to say トムは英語を、太郎は日本語を、フィリップはフランス語をそれぞれ話した to avoid ambiguity.

I see. I also noticed the ambiguity in the second sentence but I thought that I was just overthinking again. Thank you very much for pointing out the different possibilities!