(a) A stage SMB/DF can in many cases not be solved through insight or theoretically (and then apply a solution) as the game does not provide complete information regarding said stage or special mechanics when first encountered. In addition for human players, that player must test various parts of the game (jump timing, path) to reach a solution.
Again - your example of a "pure trial and error" game would not be a difficult one, merely an exercise in patience.
(b) That quote feels out of context. Game flow was mentioned to note that gameplay that involved trial and error where error led to a failure state are not inherently detrimental to an experience. I made no correlation between game flow and difficulty.
Solving Blighted Bouquet wouldn't have taken too many tries. First time seeing BB cast, someone stuns/silences/pushes and the tank pops mitigation but everyone dies. We know we can't interrupt it and that it hits everyone. When BB is first used, there are only briar patches in the arena and no adds, so you might try to have the boss on it or stand on it to mitigate the mechanic but everyone dies. Or you'd try standing far away or under the boss but everyone dies. But some of the attempted solutions will involve a player not doing anything (such as a melee DPS at max range waiting for the cast) and that player survives. Now there's a partial solution to the puzzle. Since BB is cast around 70% and the initial burn of the boss is quick, the process of reaching BB and testing it isn't very tedious.
(c) The Titan example was mainly to highlight the failure feedback loop and what is perceived as fair difficulty. That aside, I get the impression that there are a multitude of one-hit kill mechanics outside of raid encounters. But I'm wary. Please detail what your definition of a "one-hit kill mechanic" is.
As for Dragon's Lair, are there not times in the game where observation and deduction can provide the player with the correct answer without blindly attempting all solutions?