are these polynomials or rationals functions?

This is response to QUESTION 1.

As Fedor pointed out, we're dealing with the Chebyshev polynomials $P_n(2\cos t)=\sin nt/\sin t$. So we must show that if $$ \sum_{n=1}^N \sin nt = 0 , \quad\quad\quad\quad (1) $$ then also $\sum_{n=1}^N \sin^m nt = 0$ for any odd exponent $m\ge 1$.

We may take $0<t<\pi/2$. Also, the sum in (1) can of course be evaluated, and we find that (1) is equivalent to $$ \cos t/2 = \cos (N+1/2) t . \quad\quad\quad\quad (2) $$ I now claim that if (2) holds for $t$, then it also holds for any multiple of $t$. To see this, we just notice that (2) means that $s=t/2$ satisfies $(2N+1)s = 2\pi M\pm s$, for some $M\ge 1$ and a choice of sign (recall that $0<s<\pi/4$). In other words, (2) requires $s$ to be a rational multiple of $\pi$ with denominator $N$ or $N+1$, and clearly this property is preserved under taking integer multiples.

Now everything is clear: $\sin^m\alpha$ can be written as a linear combination of $\sin j\alpha$, with $j$ odd, and we have just seen that (1) implies that also $\sum_{n=1}^N \sin njt = 0$ for any odd $j$.

Question 2.

Since $P_n(2\cos t)=\sin(nt)/\sin t$, we get that the roots of $P_n$ are $2\cos(\pi k/n)$, $k=1,\dots,n-1$. That is, a number $\kappa=2\cos(\pi a/b)$, $0<a<b$, gcd$(a,b)=1$, is a root of $P_1\dots P_n$ of multiplicity $[n/b]$. We have to prove that $\kappa$ is a root of the numerator of your fraction of multiplicity at least $[n/b]$. Fix odd integer $r$ and consider the sum $$S_{rmn}(t)=\sum_{j=m}^n \sin(rjt)=-\frac{\sin\frac{n+m}2 rt \sin\frac{n-m-1}2 rt}{\sin \frac{rt}2}.$$ As Christian explained, the $m$-th multiple in the numerator is a linear combination of $(\sin t)^{-1}S_{rmn}(t)$ for different odd $r$. We see that if $n+m$ or $n-m-1$ is divisible by $2b$ (note that both divisibilities are not possible for the same $m$), then $S_{rmn}(\pi a/b)=0$. The total number of such $m$ is the total number of multiples of $2b$ in the set $\{-1,0,1,\dots,2n\}\setminus \{n-1,n\}$. It equals $1+[2n/(2b)]-\delta\geqslant [n/b]$ as desired (here $\delta$ equals 1 if $n-1$ or $n$ is divisible by $2b$, else $\delta=0$.)