Pay attention to the fact that in the formula in $\,(1)\,$ , for any $\,1\leq i, the denominator $\,j-i\,$ is always positive , so what "decides" the sign there is merely the numerator $\,\sigma(j)-\sigma(i)\,$ , thus: both "definitions" you wrote down of sgn are exactly the same.
Now, let us write down the given information: $\frac{p\left(x_{\sigma(1)},...,x_{\sigma(n)}\right)}{p\left(x_1,...,x_n\right)}=\frac{\prod_{1\leq iand as before: the factors in the denominator are the very same factors in the numerator up to sign, and there's a change of sign, say for some pairs $\,(r,s)\,,\,(i,j)\in P\,$: $\frac{x_{\sigma(j)}-x_{\sigma(i)}}{x_r-x_s}=\frac{-(x_r-x_s)}{x_r-x_s}=-1$iff exactly the same change exists in $\frac{\sigma(j)-\sigma(i)}{r-s}=\frac{-(r-s)}{r-s}=-1$ so from here clearly we have that $\,\,(**)=sgn(\sigma)\,$
Finally, we can try to attack $\,(3)\,$ with the following:
Lemma: For every permutation $\,\sigma\in S_n\,\,,\,n-N_\sigma=T_\sigma $ , with $\,T_\sigma:=\,$number of transpositions $\,(i \,j)\,,\,1\leq i\neq j\leq n\,$ , needed to express $\,\sigma\,$ as a product of transpositions.
Proof: It is enough to prove for cycles, as any permutation can be written as a product of (disjoint) cycles: let $\,\sigma = (i_1\,i_2\,,...,i_r)\,$ be an $\,r-\,$cycle , then $\sigma=(i_1\,i_r)(i_1\,i_{r-1})\cdot...\cdot (i_1\,i_3)(i_1\,i_2)$and we can see that the number of transpositions in the right hand side is $\,r-1=n-N_\sigma\Longleftrightarrow N_\sigma=n-r+1$ which is clearly the number of orbits of $\,\sigma\,$ , as it fixes $\,n-r\,$ points in $\,\{1,2,...,n\}\,$ Q.E.D.
Well, then we already are there since any transposition has sign $\,-1\,$, obviously, so that $(-1)^{n-N_\sigma}=(-1)^{r-1}$and let us not forget that the number of orbits of $\,\sigma\,$ is just another name for "the number of different (disjoint) cycles in the decomposition of $\,\sigma\,$"