So, the form with bat focuses on the numeral and it's not really an undetermined article...but in the last centuries, due to the Romance influence, bat tends to assume the role of an undetermined article...so, the usage and meaning of bat in this context is variable....I'm correct?
that's more or less correct. I wouldn't go to say that "bat" is not an undetermined article. I'd rather put it this way: Basque is less restrictive when using undetermined articles. Consider the example of the fox:
"What are you? " "I'm a fox"
English chooses to use an undetermined article... even if it is not necessary: it is very clear which fox it is!
So Basque would take the -a form:
"Ni azeria naiz"
(bat form being also possible).
But when the indetermination is more clear, like in
"A fireman was here"
where you clearly don't know which one, you'd translate as:
"Suhiltzaile bat egon da hemen"
Said that, it is also true that popular Basque would preserve the -a for certain nouns, even when the indetermination is clear:
"Azeria ibili da oilategian"
You don't use the "bat" form, even if you don't know the fox that has been eating your chickens, but this is more of a custom that you'd use for nouns as "otso", "hartz".