Each time you're parried, you do NOT get an extra swing from the boss. On bosses that have parry hasting enabled (usually not the harder hitting bosses), if you are parried AND fall within a small window of time between the boss's swings, it will make the next hit come a little bit faster. It will not, however, give him an extra hit.
#2 - Expertise reduces the parry and dodge factor that hit doesn't cover once you hit the hit cap. Each parry that lands means an extra swing or two from the boss and you taking additional damage as a penalty. So think of it as the "other hit cap" half. 26 Expertise cuts down 6.5% of the enemy's dodge/parry.