Completely agree. We often set our selves up for failure before we've begun by expecting to fail. I wish people would expect to succeed more. Because they honestly would.
That's my favourite part about the dog training levels, is that I can keep trying them over and over to improve my strategies on levels... except for the last one of set since once you beat it, you move on lol. I think for me it started with the chest levels. The rewards for the chest levels were so poor that using any coins or boosters meant I was actually losing even if I was winning. So I started playing those levels with the strict rule that I had to complete them without using coins or boosters (sometimes I cheat and use a shovel, rarely... but it has happened). Come to think of it, I've probably used a shovel or two on the dog levels here and there as well. So I guess I'm a giant liar when I say I do them booster free.
It's interesting that you experience such randomness here and there with the behaviour of fruits, boosters and other level aspects. I will admit that on the rarest of occurrences I have experienced a result that was unexpected. I can probably count on two hands the number of times I've felt that way but they do exist. Often something will happen, like a fruit will disappear when I don't expect it to but then I realize, oh I cleared a gnome and when that happens it clear all the fruits on top of the gnome. Sometimes I'll watch fruit fall in from the top and it looks like I should create a bomb or an even better power-up but instead they'll land in such a way that it just does a couple match 3's and I get nothing. I know that if I'm making matches by firefly pots and I make two matches at the same time on the same pot I'll only collect one firefly. The problem with situations that don't work out like we plan or expect is that they're really hard to duplicate. I'm going to try to duplicate your rainbow blast with two fruits on the same pot collecting two fireflies situation. It'll take some time but I'll see if I can replicate your experience.