Neither "And a god was the Word" nor: "And God was the Word" are correct translations for θεὸς ἦν ὁ λόγος.
To understand the implications of the last clause, you need to understand something of Greek syntax. First, Greek distinguishes the role a noun plays in a sentence by changing the case. In general, if the noun is the subject, it is in the nominative case. If it is the direct object, it is in the accusative case. However, there is a strange class of verbs that do not take a direct object, instead, they take a predicate. There are three verbs that do this in Koine Greek. This means that you have two nouns that are the same case (nominative), where one is the subject, and one is the predicate. So, if both are in the same case, how do you know which is the subject, and which is the predicate?
Here are the rules:
If both nouns have the article attached, then the first is the subject, the second is the predicate.
If NEITHER noun has the article attached, then the first is the subject, the second is the predicate.
If one has an article, but the other does not, then the one WITH the article is the subject, and the one without the article is the predicate.
So, in the phrase "και θεος ην ο λογος", we see that λογος has an article (ὁ) and θεος does not. Thus, ὁ λογος is the subject, while θεος is the predicate.
When translated into English, because λογος is the subject, it must be placed first. English has syntactical rules that must be followed as well. So, the only valid translation of θεὸς ἦν ὁ λόγος is "and the Word was God."
Simply because a noun is anarthrous (not preceded by an article) does not mean it is indefinite. John uses a number of anarthrous nouns in his prologue including θεος yet none are assigned the indefinite article in translation and correctly so. It must also be noted that in Greek there is no such thing as an indefinite article.
First, Koine Greek normally drops the article in a prepositional phrase. The absence of the article in a prepositional phrase is normal and doesn't mean anything. It is the INCLUSION of the article in a prepositional phrase that is unusual and thus means something.
The prepositional phrase "εν αρχη" for example, does not contain an article, but is still properly translated "in the beginning." The prepositional phrase "προς τον θεον," however, does include the article (τον). Since it would have been grammatically proper not to include it, then the INCLSION of the article here means something. In general, the inclusion of an article in Greek when it is not expected means the writer is being specific.
Bill Mounce (who I believe quotes from Wallace) makes this observation in identifying the subject from the predicate in the nominative case.
"In English the subject and predicate nominative are distinguished by word order (the subject comes first). Not so in Greek. Since word order in Greek is quite flexible and is used for emphasis rather than for strict grammatical function, other means are used to distinguish subject from predicate nominative. For example, if one of the two nouns has the definite article, it is the subject. A good illustration of this is John 1:1c. The English versions typically have, “and the Word was God.” But in Greek, the word order has been reversed..... We know that “the Word” is the subject because it has the definite article, and we translate it accordingly: “and the Word was God.”
It has been brought to my attention that I have not yet addressed the major point of the question of the poster. For this I apologize. As a disclaimer, I have no skills with Latin so, I am certainly open to correction on this score. However, I do know a little about the Douay-Rheims Bible. The Douay-Rheims Bible is devoted to the Latin text. If you will read the preface to the Douay-Rheims Bible it is quite clear that the translators had little regard for the original language text. They felt that the Greek text had been corrupted and the Latin text had not. It must be understood that the Latin text is nothing more than a translation of the Greek text. For one to use the Latin text (or any other translation) as a standard for creating a new translation is most irresponsible. What we wind up with is nothing more that a translation of a translation. In the 1582 publication of the Douay-Rheims translation, the clause is translated, "and God was the Word." While the sentiment may be correct, it is not an accurate translation of the Latin text. In the later publications of the Douay-Rheims, et Deus erat Verbum has been corrected to agree with both the Latin and the Greek - "And the Word was God."