Epidemic just seriously needs to be toned down in WvW. It's got a 20 second cooldown for crying out loud. Even if you took away 80% of it's effectiveness and only let it transfer 5 of each condition, it's so much better than any other selection to make for necro, because 5 of each condition every 20 seconds spread to 5 players is HUGE, let alone 25. We'd all probably continuing to use epi anyways.
I don't see much concern about the cast time on it. Nobody can really tell when we're casting it anyways because the animation isn't massive. As a side note: a debuff on the buff bar is a bad way to go about that. Considering most people have also sorts of hero's banners, food, oil, and profession specific buffs across their bar, it's near impossible to quickly identify all the buffs/debuffs you're getting that aren't the yellow buffs or red conditions. It also means that you're playing the UI rather than the game, which is something GW2 likes to avoid.
At this point, I really just don't get why epi isn't an elite skill. It's so build defining as is, and is so absurdly far above and beyond the best choice in WvW. It was fine as it was in PvE anyways.
If you want counterplay, you need to change and lengthen the cast animation to make it way more obvious. It's rare that I find myself getting interrupted when casting epi, and once I've finished casting, "counterplay" is a joke, because it's either spamming tons of condi clear and hoping you clear the damaging condis, otherwise you die, or to use resistance. Resistance isn't all that great either considering how easy it is for necro to corrupt boons.