is it true that なら is merely a short form of ならば and as such, both are totally interchangeable without affecting the nuance of the sentence?
Also, a second question is is ならば more "formal" than "なら" ?
First things first, なら does come historically from ならば. ならば itself is the hypothetical form of the old copula なり, and it was constructed in the same way the auxiliary verb たり changed into たらば, as I've already explained here: Can 【～たら】 be a short form of 【～てから】?
So it's なら (the mizenkei of なり) plus the hypothetical particle ば. Originally, it was just the hypothetical form of the copula, so you could say it was the equivalent of hypothetical and conditional forms of the modern copula (だ) such as であれば and だったら. But in modern language it became a fixed expression and was further shortened to なら in most cases, and came to be used with verbs as well as nouns.
Are なら and ならば completely interchangeable?
As far as I know yes, but the nuance of the sentence could change since ならば is not only more formal than なら, but also has a somewhat literary feeling to it. It's pretty much absent from everyday speech, and even in very polite speech I've rarely ever heard it (which might be just my experience, but that's all I have). So if you want to be safe, you'd better use なら everywhere until you get a better feel for the language.