1. My issue with this is not that the mechanic exists, but that it is impossible to tell that this is what you're walking into in this game without mousing over the mob first. The graphic under the mobs is generic and there's as far as I know no obvious graphic representing specifically fenumal presence. You can say whatever you want, but any form of massive damage should be telegraphed in some way whether it be a noticeable cast or a noticeable graphic on the mob specific to that ability, anything else is cheap and poor game design.
And you can say what you want about "slow down then on HC and mouse over every mob!" but that is stupid and you should feel stupid for suggesting this instead of GGG just putting a noticeable graphic specific to this ability.
2. Not really, I mean, we don't have a combat log. So there's no way for me to know they do chaos damage unless I find a pack, take some damage, retreat, change up gear, and see what they are doing. All I know about them prior to poedb is that they have more HP and hit harder, what type of hit, how that's affected, and other things like the fact that it's a chaos dmg add are not really recognizable without significant testing by someone or a data mine.
3. Been playing HC for 2 leagues now and i really start to understand what people mean by "PoE is not a game for HC". Every death i experienced since playing was BS type one like dying to white mobs in t2 map with 7k ehp (because i went from one location to another in burial chambers and my character appeared in the middle of 50 kitava heralds with prox shields) or getting one shot. I really do enjoy the time when i play through 10 acts with my first character of the league, doing all te bosses gives me adrenaline as i have to deal with mechanics and i know i can die any moment. Entering maps always kills it for me.