Dear Bero,
I agree with you on the main line: I mean, I think too that venom has an evolution history strictly related to paralizing and also digesting preys, and not as a defence system... but the question is: could the "defence" had played a role (maybe a minor role) in the evolution of venoms in snakes or in some snake species?
That said.
I've read reports about snake bites that are immediately painful: I do not remember which species were involved, where and when, but I'm pretty sure about that.
On the contrary I've a monography about Taipan bites in Australia, with many human bite cases and pain was absolutely absent... I do believe Taipan bites are painless, but it's not true that a human (or a pig or a dog) can crash a Taipan head in a few seconds: a Taipan could deliver 2-3 bites very rapidly and disappear in a fraction of a second, just like many european whipsnakes (viridiflavus, caspius, and so on) have disappeared under our sight in a fraction of a second!
The questionaire is meant for people, you write: but this is obvious. You cannot interwiev a dog or a horse and ask it, if the snake bite was immediately painful or not!
You write that sheep are always bitten by snakes, and dogs too, but this means nothing. Humans always die in motorbike or car accidents, but this fact does not mean humans don't know the risks of driving a car or a motorbike!
This fact would mean more if a dog (or a cow) already bitten by a venomous snake (and having they clearly seen the offending snake!) would ignore in the future the presence of a similarly well seen and exposed snake! But this is almost impossible to prove, considering also the fact that in natural environment dogs or sheep are very often bitten by snakes inside the vegetation, without they know what's happening or without they even see the offending snake!
To summarize: I think almost the same as you, but we could not exclude that, maybe only in some snake species, a painful venomous bite could have played a (probably minor) role in venom evolution, just like it happened in wasps and bees, whose sting is always very painful...
I was only once bitten by a venomous snake: a baby V.aspis managed to put one of its fangs inside one of my fingers. No pain.
I was bitten many times by non venomous snakes, and big colubrid bites can be rather painful (just like being scratched by thorns): but the same bite in a horse, or a wild boar or a big dog would mean almost nothing as regards pain, I suppose...
But, as you correctly said, this is the only reaction small/normally sized snakes (I mean: not huge constrictors) can do...
Another interesting question: snake instinct and "awarness"...
Snakes act and behave by instinct...ok... (I would not say that alien creatures could say the same for us humans....
)
A Taipan knows nothing about its venom: but it "knows" very well that after a bite it can release the prey and it "knows" very well that it will soon find it dead and ready to be eaten. An Aesculapian snake, on the contrary, "knows" that the prey must be coiled up at once with the body if it has to be eaten... otherwise, prey is lost!
How can we say with certainty that a Taipan "does not instincly know" it has a terrible weapon also on the defence side?