You almost had me. I was almost ready to agree with you that maybe Spy isn't so bad if you Spy-check (ready enough that I didn't respond for ten days). Indeed, in my own games this discussion has helped me become a better checker. But then I saw this.
Watch this entire video, without skipping to another tab, without even pausing because I'm that angry, and tell me the Spy is a fair and balanced class. Look at 9:00. Practically the whole team had gone Sniper and Pyro in their rage, they were flaming everywhere with the objective forgotten, but he was gone, and kept returning for stab after stab. You simply cannot shoot at the enemies guarding/pushing the objective in front of you and the Spy behind you at the same time, unless you have a dedicated Spy-checker who will be otherwise useless.
ArraySeven uses the Blutsauger in public servers, and the Crusader's Crossbow in competitive play..
"And fighting is dangerous", so run. Fire a couple of shots and then cloak. You will get away, and if by some miracle you don't, you will cause paranoia. "Sometimes, he'll be facing behind his buddy just to watch out for spies": good, he's a smart Medic, but he's only delaying the inevitable. Wait until he turns around to heal that burning Demoman. I cannot stress this enough: they will always stop checking their backs.
By "out of sight", I implied "out of hearing range", or "at a time covered by the sounds of battle".
Although I can see that the effectiveness of Your Eternal Reward would be hampered by communication, I think it would be about as effective as the other knives after the first backstab. Does it not bother you that excellent communication and supreme effort are required to counter Spies?
I think melee is useful against the unskilled Spies I commonly encounter, and maybe if you can stay outside Spy melee range with the Demoman's swords and the Disciplinary Action. Otherwise, any Spy who can trickstab will indeed 100% matador stab anybody who tries to melee him. See here. It is dangerous to come within melee range of a Spy, regardless of skill. I am not a very good Spy, but I have performed a matador stab accidentally.
The skill required to be a good Spy is right-click, 4, (1-9), hold w, turn, wait, right-click, hold w again, click until you're caught, right-click, hold w, repeat. The rewards are instant kills. Instant kills. I can't scream through a computer; you'll have to settle for bold. Even if I get just 1 kill before I'm caught and killed, I am satisfied, because that's an enemy that would have taken my team a lot of firepower to bring down, whom I killed with a single click, and I caused distraction, paranoia, and anger along my way.
But suppose you can't even manage that. I've heard tell of a Spy camped out near the enemy with the Cloak and Dagger, relaying enemy health and Übercharge to his team. (Imagine that, a Spy doing actual Spy instead of behind his normal overpowered instakill self.) This takes zero skill and has a moderate payoff in helping your team.