I can't say, as I don't know how the block rate is calculated. It did seem to be weird to me though, I often blocked A LOT more than 20%. blocking every 1/5 hits would be fine, however this isn't the case People block a lot more than once every five hits.
As for the adding skills thing...Anyone can use thorns or Unity etc. There are a lot of unfair builds, and things that don't seem right. The best you could do is try and balance them better so that it's more smooth.
I think that manashields duration is what makes it Over Powered. Like you said people with leach could have a perma manashield. The problem is though, when you balance something just for PVP, then PVE gets unbalanced. You would really have to have 2 separate effects, one for PVP and one for PVE.
In PVP Manashield would last 8 seconds total and have a cooldown of 30 Seconds.
While in PVE it would have a duration of 25 seconds and a cooldown of 35 seconds.
The main problem I see with manashield is people with leach, I'm not quite sure how you would go about fixing that though, Most rampagers or melee chars rely on Leach Very heavily.