It's a fallacy to argue that you need that level of precision to have a valid scientific theory. You're grasping at straws.
If you have two theories about a rigged roulette wheel and the best one is accurate 4% of the time and the next best is accurate 3% of the time then you have Terry theories that are both better than the null hypothesis, and you use the 4% theory try place your bets.
Or to put it in terms a statistician would, you can have statistically significant parameters in your model even though the residual is very large.
The usual interpretation of a large residual is that your parameters you have modeled are correct but you haven't yet included ALL the parameters. Further investigation should add more parameters.
In plainer English: human caused global warming is real but there are a lot more additional factors that impact climate that we haven't understood.
I do not not know your definition of "Valid Scientific Theory", english is often imprecise and lacking when talking about precise things. If a valid scientific theory to you encompasses relativity, newtonian gravity, quantum mechanics and evolution then AGW does not belong in there. If your Definition includes; string theory, many worlds theory, and we are living in a simulation, then AGW belongs there.
Science is "You have to be this tall to ride on the rollercoaster" it is not "you have to be taller than the guy next to you to ride the rollercoaster".
I do not know why you keep using the roulette wheel example, it is retarded, anyone with a brain would not place a bet yet you contend that the wise thing to do is to place a bet on the least shitty of the shittiest theories.
If AGW is proven with the same vigor as other scientific theories, then to displace AGW you would need an even better theory, in that case then it would be right to ask someone to submit a better theory in order to disprove AGW. AGW is asserted as a scientific truth/theory without the prequisite proof, as such it can be dismissed as a scientific truth/theory without prequisite proof.
Imagine if every theory needed to be proved wrong by a better theory, How did life on earth arise; by alien jizz of course unless you have a better theory to disprove it. How did the universe begin; by god of course unless you have a better theory to disprove it. This can go on and on.