As someone who double-charmed on Mangler - the minimum 5% chance to break per tick most certainly does not exist. I think your comment on the min/max mix up are likely correct. Druid charms at max level last about 15 minutes - following basic probability, this would mean I have a 0.95 ^ 150 = 4.5 e-4 or .00045 or .045% chance of lasting full duration - or 1-in-2,222!! I've seen that spell last full duration many times and I can promise you I didn't cast it 2000 times. Statistical anomaly? Yea...and Jeffrey Epstein killed himself...
A dwarf with base 55 CHA does get a noticeable degree of critical Lull/Soothe failures however. There is probably a minimum CHA worth having, something like 75 or 80.