My bolds for emphasis. If you can't deal enough DPS in a timely manner, that is the exact definition of "perhaps you should be playing on a lower difficulty level."
But this is the only case of those you provided where having a more capable character would allow you to work around the issue.
The bottom line seems to be: Does it suck that this happens occasionally? Yes. Are the devs aware that this happens occasionally? Yes. Your choices seem to be to play those quests you have identified on a lower difficulty level, or to not play them at all, or to deal with the cases where you run into this situation. Your in game choices, that is.
Out of game it looks like you're doing all you can also: You're making the community aware of the issue and perhaps keeping the issue on the devs radar. But history shows that issues like this one, which have a distinct 'work around' that a player can implement via their in game choices, have rarely been on the top of the devs list for spending time fixing them. Love, hate, or do not care about that reality as you see fit, but it does seem to be the case.