You've misunderstood the circumstances of this case. Of course you can be too drunk to consent to sex, the judge's ruling doesn't effect that at all. What was impossible to determine in this case is how drunk the complainant was at the time when consent would've been given. Lacking the complainant's memory of the events, or any relevant evidence, the judge couldn't convict.

Legal precedence only goes so far in matters like this. The courts have recently ruled that their approach to sexual assault had to change, mostly for the protection of the victim. So now the foundation law concerning consent and sexual assault has been established by the Supreme Court of Canada. Legal rulings which conform to that will be upheld. Legal rulings which don't will be struck down.

Prior allegations of misconduct wouldn't normally be accepted as evidence in a criminal trial.