PureNoob, first of all you have too many boxed turrets. A boxed ret means a ret whose line of fire is blocked by something else: other structures, mostly other turrets.
The ideal placement for turrets varies for different locations and situations, but in this case you want to think of how the aliens come in the base, which is entirely from the ledge. They may jump the ledge or come up the ramp, doesn't matter. What matters is you have a box with 5 walls and aliens coming from where the 6th is missing. The box is low and wide. The best turret layout in such a case is a line of them, stringing left to right and hugging closely the structures behind them (well, not too close, see more below).
Yours are not in a line, they are scattered around. Sure, the 5 in front are ok, but you are completely wasting the other 2. The one on the left is pretty badly boxed between those two others and the armory. The one in the back is boxed by the RC. They will almost never get to fire.
Now, about RC hopping. To place rets that "see" the top of the RC you need to consider very carefully the lines of fire. Practice in devmap, place rets, then turn goon and see which ones can fire at you. In your setup, I'm afraid that only 1 (one!) ret sees the RC top, the one in the middle of the line of five. Maybe the one on the right of it, but it may be too far. Obviously, just one ret is not enough, a goon will have a few good seconds in which to chew your RC.
And I fear that a mara, being smaller, may evade even that ret altogether, if he pulls back in the corner. Because the default RC placement on ATCS is quite bad, allowing a very nasty blind spot at the top, which is why most people try to move it back all the way into the corner ASAP.
And to answer your question, the turret behind the RC is mostly useless. It does not see the top of the RC. You have to realize that no matter how the models look, all things in trem are actually rectangular boxes. The RC may look like it's smaller at the top, but in reality the bounding box is as wide as the bottom all the way to the top. Think of it like a big crate.
You need a fair amount of practice as builder (or just spectating and observing turrets) before you start to accurately assess where to place turrets so they can cover certain places. If you place them too far they don't fire, if they're too close they can't "see" past the edges of the "crates".
And you have to realize how truly frightening a skilled goon can be. If he walks on top of your turrets (which have trouble tracking over-head) and never stands still, he has about 5 seconds of roaming around. That's time enough to get both nodes and probably arm too, depending on skill and luck.
So what you want to do is make sure as many turrets as possible hit the goon in the only moment he's fully exposed, and that's the moment he comes in over the ledge, into the box. A side-to-side line of turrets will do that, will deal maximum damage in the split second he needs to come in.