First-order Contribution to the Self-energy Operator

In Altand and Simons’ book ‘Condensed Matter Field Theory,’ on page 225 they claim that the first-order contribution to the self-energy (effective mass) operator reads
$$big[Sigma_p^{(1)}big]^{ab} = -delta^{ab} frac{g}{L^d} Big(frac{1}{N}sum_{p’}G_{0,p’} + sum_{p’}G_{0,p-p’} ) $$
where the first (second) term corresponds to the first (second) diagram. Here we are dealing with a $N$-component vector field $phi = {phi_1, cdots, phi_N}$ under the action
$$S[phi] = int d^dx Big(frac{1}{2}(partialphi)cdot(partialphi) + frac{r}{2}phicdotphi + frac{g}{4N}(phicdotphi)^2 Big). $$

I have two questions on this:

(a) The expression suggests that we have a factor of 4 for the first contribution, and $4N$ for the second one. How can one get such factors systematically (especially for the $N$ factor)?

(b) For the second term, can I write it as
$$sum_{p’} G_{0,p’} $$
which to me is simply a shift (plus reverting the ‘direction’ of summation) of the dummy variable and has no effect for an infinite sum. Also, from diagram’s perspective this just looks like assigning $p’$ to the straight line instead of $p-p’$. If this is true, does it mean that the first-order contribution to the self-energy operator is actually independent from momentum $p$?