Re: Oliver surviving
Long story short, it is unrealistic and - statistically speaking - his probability of survival is really, really low. However, this is fiction at the end of the day and - much like in other aspects of the show, and in other shows - suspension of disbelief is often called upon. YMMV whether this exceeds one's particular threshold or not and why. This unlikely event in and of itself is hardly unique; both within the show and within the context of fiction as a whole.
Anyway, let's break things down.
Let's start with the stab wound. Given Ra's stabbed Oliver in the right upper quadrant of his abdomen, the biggest risk in this region would be trauma to his liver which is likely going to bleed profusely. However, to nitpick the show, this was not portrayed accurately at all as it should not result in hematemesis/vomiting blood or hemoptysis/coughing blood. A major bleed from liver trauma would cause the liver to bleed into the abdominal cavity. It's far less likely for it to collect and backtrack its way up the GI tract - through the hepatic duct, through the bowels, into the stomach and up the gullet to the mouth. And shouldn't cause blood to collect in his lungs or trachea at all. However, internal bleeding is far less visual. No, the blood from the mouth was done because it just makes for dramatic imagery, to show that it's a major injury and because it looks "cool". Even if the blood was to collect in latter way, it would be more realistic for it to be gushing out from Oliver's rectum rather than his mouth; which would be less cool looking.
Next, we move on to the fall. This, I feel, is actually somewhat more survivable. A big factor of the severity of a fall injury is the speed at which it occurred in (or to be ultra precise, the rapid deceleration that occurs at the time of impact); meaning a slower fall tends to be a less serious fall all else being equal. While it's hard to say with any degree of certainty, I don't think that Oliver was falling at terminal velocity (the maximum speed a person call fall in a given non-vacuum environment). It takes about half a kilometre's drop to achieve this and I don't think Oliver fell from that height. But even if this occurred, the odds of survival is still far from zero. There have been numerous reports of people surviving falls at terminal velocity. Given the snow-covered terrain and possibility of a broken/staggered fall, this was probably the less deadly of the two traumas Oliver sustained.
Finally, let's talk about exposure. Given that he's unconscious, without protective layers, possibly in hypovolemic shock (from the aforementioned blood loss) and left exposed for several days, this is probably the one likeliest to kill Oliver - albeit over a period of time. Oliver would be at a high risk of hypothermia and frostbite (especially if he's in shock and thus peripherally shut down). Yet interestingly, this is the one where only a few people take issue with.
Overall, it should come as no surprise that fiction need not be realistic. What it needs to do is tell a good story. With that comes the need to take artistic license with the science - downplaying the severity of some things as necessary and exaggerating the severity of others when it suits them. All in the name of generating good tension and drama. Meaning that Oliver should be seriously injured and would need time and help to fully recover, if at all.
