This ability has worked fine for me while soloing. I always use it from melee range. I know its pointless while soloing but I have tried it several times and it works every time so far.
At level 30, not sure what level we get it, the Feign Death 2 has a refresh of 4 seconds. Putting both FD 1 and 2 on my bar would not net me more usage. The seem coded that using one resets the timer on the other (even though the icons may be lit up you can't use them until the timer expires).
As far as higher level mobs resisting it, I found out last night this is a big fat YES. Several deaths against much higher level mobs to me, and not once did FD save me. My FD was successful and I got the everyone thinks you are dead message, but they kept right on kicking my ass.