What is the correct notation for a multivariable function?

Technically speaking, you are correct. If I have a function $f:\mathbb{R}^2\to\mathbb{R}$, and I define $p\in\mathbb{R}^2$ to be the point $p=(1,2)$, then logically, there shouldn't be any difference between writing $f(p)$ or $f((1,2))$; and indeed, there is no logical difference. However, $f(1,2)$ is a convenient shorthand, because the extra pair of parentheses provide no added clarity.

I might also add that, in some places in mathematics, it is common to denote the application of a function without parentheses; for example, if $T:V\to W$ is a linear map from a vector space $V$ to a vector space $W$, you'll often see $Tv$ written for the image of an element $v\in V$ under $T$.

Choosing when to distinguish, or not distinguish, things that have "canonical" identifications, such as ordered pairs $((a,b),c)$ and ordered triples $(a,b,c)$, is an important detail in a lot of mathematics.

Yes, it’s formally correct. In most contexts, however, the added notational burden serves no useful purpose and merely makes the mathematics harder to read, which is contrary to one of the most important functions of good notation. In most calculus settings, for instance, it makes at least as much sense to think of the two arguments of $f:\Bbb R^2\to\Bbb R$ as independent entities $x$ and $y$, not as elements of a single compound entity $\langle x,y\rangle$. There are times, however, especially in set-theoretic contexts, in which it really is important to think of an input as a an ordered pair or $n$-tuple, and then it helps to reflect this in the notation.