I don't know enough to answer the question for the principal branch of the argument or the question about $R(T)$, but the value distribution of $\DeclareMathOperator{\arg}{arg}\arg\zeta(1/2+it)$ is extremely well-studied in the literature.

First, following the literature in the subject, define, when $t$ is not the ordinate of a non-trivial zero,
$$S(t) = \frac{1}{\pi} \arg\zeta(1/2+it) = \frac{1}{\pi}\Im\int_{\infty+it}^{1/2+it} \frac{\zeta'}{\zeta}(s) \,ds.$$
Here the integral is taken on the horizontal line from $1/2+it$ to $\infty$. When $t$ is an ordinate, the value of $S(t)$ is a matter of convention, so one takes it to be $\frac{S(t^+) + S(t^-)}{2}$.

Note that $\log\zeta$ is defined exactly analogously, except that one does not take imaginary parts and there is no normalizing factor $1/\pi$.

As you might know, the Euler product of $\zeta(s)$ is equivalent to the assertion that
$$\log\zeta(s) = \sum_{n\geqslant 1} \frac{\Lambda(n)}{n^s \log n},$$
for $\Re s > 1$.

It's straightforward to see that this representation does not make sense in the critical strip $\sigma \in (0,1)$ since the right hand side doesn't converge even conditionally. Having said that, when considering statistical questions on the distribution of $\zeta$, the Euler product still exerts its influence -- see, for example Principle 1.3 in Harper's Bourbaki seminar.

In particular, using a weighted version of such a statement, Selberg showed that
$$\frac{1}{2T}\int_{-T}^T S(t)^{2k}\,dt = \frac{(2k)!}{(2\pi)^{2k} k!} (\log\log T)^k (1+o(1)).$$
Further, since $S(T)$ is odd,
$$\frac{1}{2T}\int_{-T}^T S(t)^{2k+1}\,dt = 0.$$
Here, $k\geqslant 0$ is an integer.

These are asymptotically precisely the moments of a normal distribution with mean $0$ and variance $\frac{1}{2\pi}\log\log T$. Since the normal distribution is characterized by its moments, this says that $S(t)$ is distributed approximately normally on $[-T,T]$. This is Selberg's central limit theorem for $S(T)$, which was first published, I believe, in the thesis of Selberg's sole PhD student, Kai Man Tsang.

In fact, Selberg's central limit theorem applies to the value distribution of $\log\zeta(1/2+it)$ on the complex plane. Informally, this states that $\log\zeta(1/2+it)$ is distributed like a standard complex Gaussian with mean $0$ and variance $\log\log T$. In other words, $\Re\log\zeta(1/2+it) = \log|\zeta(1/2+it)|$ and $\Im\log\zeta(1/2+it) = \pi S(t)$ behave like independent real Gaussians having mean $0$ and variance $\frac{1}{2}\log\log T$.

Note that since we are on the critical line, the primes are not sufficient to answer statistical questions about $\zeta$ (unlike the case for the Bohr-Jessen-esque results for fixed $\sigma > 1/2$, see Chapter 3 of Kowalski). Roughly speaking, the way one proceeds is to use a zero density estimate to control the contribution of zeros, thereby reducing the question to the statistical distribution of the Dirichlet polynomial
$$\sum_{p \leqslant T} \frac{1}{p^{1/2+it}},$$
This is the harder step. The easier step is to now recall the fact that for $2\leqslant p\leqslant T$, the maps $t \mapsto p^{-it}$ behave like i.i.d. random variables uniformly taking values on the unit circle $S^1 \subset \mathbb{C}$ (this is a consequence of Kronecker-Weyl together with the fundamental theorem of arithmetic; see Principle 1.1 from Harper's Bourbaki seminar). Then, the usual central limit theorem tells you that this Dirichlet polynomial converges in distribution to a complex Gaussian with mean $0$ and variance $\sum_{p\leqslant T} \frac{1}{p} \sim \log\log T$.

Other distributional questions about $S(T)$ have also been studied. Littlewood showed that the bound $S(T) = O(\log T)$ can be improved to
$$S(T) \ll \frac{\log T}{\log\log T},$$
under the assumption of the Riemann Hypothesis. Up to the quality of the implicit constant, this is still the state of the art. I believe the world record is
$$S(T) \leqslant \left(\frac{1}{4}+o(1)\right) \frac{\log T}{\log\log T},$$
due to Carneiro, Chandee and Milinovich, improving the previous record of Goldston and Gonek by $1/2$. There were earlier works on this bound (Fujii, Karatsuba and Korolëv, ...) which I will leave to the mathscinet reviews to explain. The $o(1)$ is explicit, and the best bound there is due to Carneiro, Chirre and Milinovich.

A related problem is magnitude of the mean value $S_1(T)$, defined by
$$S_1(T) = \int_0^T S(t) \,dt.$$
I think this was initiated by Littlewood also (op. cit.), who showed that on the Riemann Hypothesis $S_1(T) \ll \frac{\log T}{(\log\log T)^2}$. The discussion of the work of Carneiro and co-authors cited above also treats $S_1(T)$ (and more generally, an iterated integral called $S_n(T)$).

Finally, $\Omega$-results about $S(T)$, $S_1(T)$ and $S_n(T)$ are also known, and have been the subject of much recent research. The introduction of this paper of Chirre and Mahatab gives a good overview of the best known results.

**Edited to add:**

There's one important part of the literature that I didn't address. One important question is that of the right order of magnitude of $S(T)$. There are basically two competing conjectures.

**Conjecture 1.** The bound $S(T) \ll \frac{\log T}{\log\log T}$ is optimal up to the implicit constant.

**Conjecture 2 (Farmer, Gonek, Hughes).** One has,
$$\limsup_{T\to\infty} \frac{S(T)}{\sqrt{\log T\log\log T}} = \frac{1}{\pi \sqrt{2}}.$$

I think some people believed Conjecture 1 for a long time mostly because, as I indicated above, that bound has not been improved under RH. Most people in the area seem to believe Conjecture 2 now, though (Farmer-Gonek-Hughes make a very compelling case via several heuristic arguments).

The theory of the Riemann zeta function. It covers a lot of discussions on the properties of the argument of $\zeta(s)$ on the critical line. $\endgroup$