Welcome to the forum…
Nope… There is no benefit of critical strike chance above 100%. Critical Multiplier by contrast is how hard a crit hits and doesnt have a limit that I recall.
Its not an ailment - its how much of a chance your char has to critically hit something…
Critical Vulnerability is an ailment that can stack on enemies 4 times - each stack increases the chance that they can be critically hit by 5%. (EDITED)
you can find a lot of this info in-game by pressing G or in AndrewTilleys Community Game Guide