There are, I admit, a few levels where dumb luck can be the only difference between 'moderately challenging' and 'catastrophically implausible'. That swing kinda comes with the nature of the gameplay though - I doubt you could avoid it without rewriting the RNG into something far more static (and less enjoyable for all kinds of other reasons). Now in the case of the original adventure where you're expecting new players, anything that could lead to a seemingly impossible challenge would a Bad Idea. But in my case where I'm familiar with the mechanics and am playing around after completing (well, 95% story) the main adventure, I'd rather set the difficulty high and have a 30% chance of instantly restarting from a nasty initial block layout than set the difficulty low and have a 30% chance of breezing through the entire level without thought. So that's why that is.
As for the restrictions etc. generally, I tweaked them so that I was able to beat most levels within 2-3 attempts. The timer in "Ocean Depths" actually feels redundant to me (other than for atmosphere) since I tended to play aggressively at first, take care to avoid just the last couple of balloons, and finish within 90 seconds if I hadn't screwed myself over. The Light-Up level did take me 22 minutes including a few minor setbacks, but it was my first and only experience with that mode ... I have no idea how you're supposed to manage it on a normal board without items, honestly.
I might do an update with individual level hints and/or a few adjustments later. I've mostly moved on to a different game now, but I'm still playing quick rounds of this occasionally.