Force, Stupidity and Obi-Wan...
First and foremost, the law - above all else, is entirely about DEFINITIONS.
Thus "Serious Bodily Injury" means something, specifically from CRS:
"...at the time it was inflicted or later, involves a substantial risk of death, serious permanent disfigurement, protracted loss or impairment of any body part or breaks, fractures or burns of the second or third degree. Serious bodily is distinguished from bodily injury, defined in C.R.S. ¤ 18-901(c), in that bodily injury requires only "physical pain, illness or any impairment of physical or mental condition."
So, it's very very hard to describe your fear of "serious bodily injury" except that while a 2 inch knife might not REASONABLY be able to kill someone under all circumstances, it is inarguably capable of inflicting serious bodily injury.
The biggest problem most people run into is in TALKING after an incident. Remember you'll be judged by the "reasonable person" theory, would another reasonable person in the same exact circumstances have perceived what you perceive as a threat to YOUR PARTICULAR safety. It might be unreasonable for me, for instance, to shoot someone who was punching me in the face repeatedly, but for a 90 year old woman to shoot someone under the same circumstances would be reasonable for her. It wouldn't be reasonable for me to fear for my life because of a few punches, but for an elderly person - it would be. So not all circumstances will result in the same decision being made to prosecute.
The test is simple, the person or persons must be in your presence (directly observable by you) and they must present a DIRECT threat to either you, or someone who is also in your direct vicinity.
So to answer your questions, by direct inference...
1. Yes you could shoot the robber. You could SEE him point the gun and you believed his use of force was imminent, and it was directed at someone who was also in your presence. It's not necessary to wait until he shoots first.
2. Yes you would be, but unlike the store robbery where you heard him demand money - you might want to make sure it's not someone who is defending their own life who happens to be a CCW holder like you, or an undercover cop, etc... Best to seek cover or concealment and get more information, like the guy pointing the gun and demanding money.
The key issues are whether or not the other person was using UNLAWFUL force against another, and whether another reasonable person would have believed the same thing. It's why regular citizens are generally limited to reacting to situations that actually occur in front of them, as opposed to a cop who might be operating under information that did not occur in front of them.
All that being said, I got constant crap from the "situation" instructors at my academy for ALWAYS drilling the bad guy who was wielding a gun. Got major crap for shooting the academy director when he came out of a tent with a glock pointing at my face. Their statement was, "Hey, he was just a drunk guy who didn't know who you were". That being said, it didn't change my reasoning one bit - I wont hesitate, point a gun at someone without a damn good reason that I can perceive at that moment and the gun will rise, aim and shoot right after I yell at you to drop your weapon. Tests have shown I can do this in 1.5 seconds, wish I were faster (some guys are). I dont' believe in giving ANY bad guy a "chance", or risking the life of an innocent because I fear prosecution. If you're well-off enough, your money should be in a trust - which makes your finances pretty bullet proof.