So, let me preface this by saying, "I'm not a professional historian, but..."
...which I know goes over swimmingly well in this subreddit, but I'm a physician (and an undergraduate history major - yay!) and I actually think I might be able to shed some light on at least some of your question. I humbly request that the mods delete this post if it's not up to par.
The practice known as "trepanning" has been around since prehistorical times. Indeed, there are well documented instances and archaeological evidence of purposely drilled holes within the calvaria. It is unlikely that ancient peoples would have known specifically why cranial decompression alleviates symptoms. I can only speculate that there may have been individuals who underwent traumatic head injuries with fractures that did better than those without fractures, and so they deduced that cranial decompression was a treatment for traumatic brain injury. Again, this is rampant speculation on my part.
I’m sure others can comment more intelligently than I about precisely when the medical establishment understood what they were doing rather than simply performing an operation for no apparent reason. The field of surgery had a myriad of problems in its infancy. First, and understandably, there was initially substantial uncertainty as to the physiology as to how exactly the body worked. As recently as the 20th century, the commonly held wisdom of things like, say, heart surgery, was that it was a fool's' errand; yet we now have techniques that make heart surgery quite routine. Another problem early surgeons must have faced was lack of adequate anesthesia and paralysis. You really do need both for surgery to be a tolerable procedure. Third, there was lack of understanding of the germ theory of disease and the need for aseptic technique until Ignaz Semmelweis (famously) and Jonathan Lister Joseph Lister (thanks for the correction /u/laertes78) (somewhat less famously) adequately described antisepsis in a way that was accepted by the general medical establishment.
What’s more is that the field of surgery used to be considered less prestigious than that of medicine. Indeed, we see this bit of historical trivia reflected in the honoraria used to describe physicians versus surgeons in Great Britain: physicians are referred to as "doctor" whereas surgeons are referred to as "mister," a practice which continues to this day. The profession that eventually became described as surgery initially grew out of barbershops, and things like draining abscesses were thought to be beneath the work of educated men. As the body of surgical literature grew, anesthetic and aseptic techniques developed, and the role of surgery became increasingly recognized for the correction of anatomically related problems, surgery has become more under the purview of "medicine," to the point at which it's considered probably more prestigious (or at least, requires more rigorous training) than internal medicine amongst most physicians.
The above discussion is to give you some background understanding as to the general evolution of medicine, and more specifically the understanding of the germ theory of disease and the need for antisepsis during interventional procedures. As to your specific question, I'm afraid I'm not able to provide much help. [Sidebar: once I'm done with residency, one of my life goals is to get a PhD in history focusing on the history of medicine, so maybe I will be able this post with more information in a mere decade or so.]
The fact that they understood the significance of trepanning even in ancient times shows that there was at least some recognition that relieving intracranial pressure lead to better outcomes. It is not outside of the realm of possibility that they understood that placement of a metal plate to help stabilize a depressed skull fracture would be of benefit. What is highly doubtful, however, is that the patient would have survived the operation. There was no understanding of the germ theory of disease, and it is a dubious claim at best to suggest that any sort of ancient or pre-industrial era surgery could have been performed with adequate aseptic technique. I suspect that many patients may have undergone the surgery (or really any surgery, honestly) and survived in the immediate postoperative period, only to die from overwhelming sepsis in the days to weeks that followed.
I hope this was helpful and within the purview of making a post on /r/askhistorians. Again I would ask the moderators to delete anything they feel is not up to the standards of this subreddit.
edit: As I speculated, people with formal training in history know this stuff way better than I do. I know you're all shocked, shocked I say! Let me repost this PM verbatim from a real student of history, as is (credit /u/hitoned):
They did have the metal plate, as described by Aulus Cornelius Celsus who was alive around the same time as the show Rome takes place. Here's the relevant passage, I don't understand enough about ancient medicine to be able to tell if it'd work well or not
To add (minimally) intelligent comment upon that passage: I'm very surprised that they understood what the meninges (the thin lining that covers the brain and spinal cord) actually were. I suspect that they didn't recognize the epidural, subdural, and subarachnoid spaces to be distinct entities the way we currently do - that requires microscopic examination - but it's nonetheless impressive that they understood the concept that depressed skull fractures were a particular form of Badness^TM.
The few pages I read show that there are simultaneously very profound insights (ie, "cranial fracture ... produces inflammation") with concomitantly profound misunderstanding (it's actually bleeding, not "inflammation," that leads to almost all sequelae of acute traumatic brain injury - though people still most certainly die from cerebral swelling/edema on a subacute to chronic basis). I suspect this "hit or miss" type of evaluation was fairly common until the advent of the scientific method and the concept of reproducibility was introduced into the scientific discussion.
edit 2: I found a secondary source (yeah yeah, I know) that's basically a brief review article regarding the history of surgical practice. It’s fairly long, but quite readable. To address some specific questions people have brought up: Relatively complicated (in my opinion) procedures were being performed by ancient surgeons such as Galen. He’s still well-regarded among physicians, and he has a deep cerebral vein named after him, too. This article talks about him performing
removal of nasal polyps, removal of varicose veins, plastic surgery for cleft lip, uvulectomy for coughing, trepanning of the skull, and intestinal or abdominal wall suture of penetrating abdominal wounds of the gladiators I was actually fairly surprised to read about the variety of procedures he did.
However, the next sentence in the article is very telling as to the hit-or-miss understanding of physiology that plagued all pre-scientific area physicians:
He is considered to have had an overall negative effect on surgical progress because of his advocacy of suppuration as an essential and beneficial component of wound healing
Even Hippocrates was "[insistent] on treatment that promoted pus formation as soon as possible in wounds caused by weapons."
The article goes on to describe the evolution of the field of surgery in great detail. There's nothing in there about mortality rates, though that data would be very suspect even if it were included. Anyway. it’s definitely an interesting read!
Citation: Ann Surg. 2006 Jun; 243(6): 715–729.