Erm... nope? Cynric already spoke on the first part so I'm gonna skip on that.
As for "completely broken and overpowered"? You do know that the buff I mentioned is one of the three right? Considering the fact that it's rarely ever fully utilize, how will it be "completely broken and overpowered" if LD is shorten by approximately a min?
As for life leech, I mentioned it should at tops, tops (meaning walking dead run for the full 10 secs before it expire) heal only up to 30~40% max HP. Which means, the DRK will still die if healers are dead or sleeping on the job. This is mainly to help lighten the healers burden so they don't need to burn their biggest cd (aka Benediction). If you think this is "completely broken and overpowered", I direct you to take another look at WAR's self-aid set which I mentioned earlier.
As for damage buff, yup, it could potentially be broken, but consider this: for argument sake, let's based this on a single-target raid boss environment where both the WAR and DRK pop their oh-shit cd and will be receiving a Benediction from WHM.
Assuming WD get the damage buff I propose (let's set it on raging strike potency; 20% damage increase), within 10 secs, the biggest possible DPS spike a DRK can deal is:
(Pre-cast) Dart Arts + Carve and Spit (450 potency), Hard Slash (150 potency) > Dark Passenger (150 potency) > Syphon Strike (250 potency) > Plunge (200 potency) > Dart Arts + Souleater (400 potency)
which sums up to 1600 potency, add 3 ticks of Salted Earth at 75 potency and 3 ticks of Scourge at 40 potency each, that will total up to 1945 potency. Multiple it by 1.2 will result in 2334 potency over 3 gcd, averaging in (give and take) 778 potency per gcd.
within 6 secs, the biggest possible DPS spike a war can deal is:
Fell Cleave (500 potency) > Fell Cleave (500 potency)
which sums up to 1000 potency, add 2 ticks of Fracture at 20 potency each, that total up to 1040. However, we need to take into consideration of pre-casted Berserk (50% damage increase), Maim (20% damage increase) and that will increase the total damage to 1872 potency, averaging to (give and take) 936 potency per gcd. And that's excluding Storm's Eye's 10% slashing resistance debuff too.
And for further argument sake, let's assume I made a mistake in the calculation (which I most probably did), and say DRK can deal higher dps per gcd within the 10 sec Walking Dead window with the damage buff I propose, do recall it is on a 300 sec cd, which the DRK also need to "die" once in order to activate said damage buff. How many chance can you get to abuse this 10 sec damage buff in a single fight? 2 times? 3 times? Seriously, how the hell is this "completely broken and overpowered"?
TL;DR: Living Dead can be easily wasted, hardly ever fully utilize (outside of static-raid, probably equip with voip) and is simply a wasted potential as a 300 sec cd for the most part.