So. Yes, holding R in the middle of a fight is viable and might be your only play in a desperate situation. You need good judgement and fast swaps, but it will save your life, as it has done for me many times. If you just got out of an AOE and are about to explode from the DOT, stop what you’re doing and vent. Maybe push some rats, get in a safer position, but you need to vent. It is entirely possible to push, quick swap, vent, switch back to block in less than 5 second without getting hit, even when semi surrounded. Learning this comes with experience, but is invaluable.
You are correct - guarding against trash does little for your overheat bar. Yet it is important to remember the reduced block cost you get during high overheat. You should look into building some more block cost reduction through items (potentially getting to 90% reduced block cost), meaning you can block basically as much as you want. There is no rush - it might take trash some time to help you get your overheat down, but it is worth it. Combine this with venting mid fight and you get what you need for managing the overheat from AOE. An aggroed SV near you can be extremely helpful for removing a big chunk of that overheat. Additionally, her lvl 10 stamina regeneration talent got buffed in 1.0.5. You get where I’m going.
It is true that on Legendary the AOE effects are quite noticeable - they do a lot of damage. However, you are exaggerating a bit you do not go from 0 to 100 in 3 second. You have ample opportunity to react as long as you are not at, say, 80%. If it is clear you’re gonna explode (and even when its not) switch to staff and vent. Remember, your green bar is a resource used to heal your HP (the overheat bar). Additionally, consider building protection against AOE and using staffs with the 20% less overheat trait.
Every time a globe rat throws, a character calls it. When you hear that ‘Globadier, throwing!’ sound cue, dodge. Troll puke is extremely easy to avoid (again, simply dodge sideways) and the only time you should be standing in it is when you absolutely have to, ie doing a fighting retreat against a mob halfway through boss. Even if you do get caught up in AOE, managing overheat is in fact possible.
And yes, the cool down talent helps, but even then a lot of the time you will not be able to rely on your ult. And once you learn how to play her, you will choose to vent instead of ulting.
Unchained is incredibly versatile. Venting is a key mechanic for her. In many ways, she is easy to play (well, maybe in Champ), but hard to master.