ok first of all root is not a mental attack, but a hold one, so core is worthless. 2nd if you have 50% chance to resist the root and lionheart adds 80% you would have 90% chance to resist not 130%.
Also mobs chance is either just bugged here or its just intended that on classic -30lvl mobs can land debuffs on you, i've been silenced with both core and orfen 3 on me by the flowers running to core and that should be ~4.8% chance at equal level, not even factoring the 25 level difference.