Critical hits are scored on 20 (d20) so it's 5% chance to crit, doubling this means you crit on 19-20 so this is 10% chance or another 5%. On 18-20 it will be 15% etc.
Extending weapon styles to 5 points will run into some problems:
1)how much WS for classes, fighter gets 5, but what about rangers, paladins, bards or thieves?
2)Overspecialisation, already fighter are heavily locked into single weapon this will only made things even worse.
3)On lower levels there isn't simply enough points, so this will be for high level characters.
Overall this is why I'm for only 2 points for each style, each style with it's own... style I guess. So single weapon - precision (+hit/+crit), shield - defense (+AC/+?), two hander - damage(+dmg) , two weapons - offense (+APR).