So, figured I'd weigh in and expand on what lolix said. I tend to agree with most of the points, but thought I'd bring up the following.
Yuyu is good. Not only is she a gold, and thus potentially cheaper, her skill awaken makes her cheaper still, and maxed out she deals 497 base damage compared to Belinda's 396 (and as a gold she benefits from Sherry's awaken ability). Not to mention, if you ever manage to get an Olivie, once awakened she'll cost the same as a silver witch of the same CR.
On leveling a second Iris vs Fedore, my personal suggestion would be to CR your main Iris, and level Fedora. The reason is that while Iris is generally better, the two have slightly different roles. Iris is more of a burst healer due to her skill, while Fedora has higher base healing (21 more attack to be precise), and once awakened is cheaper to deploy.
On the one about leveling CC'd/AW'd units to 40 to 50, I would specify that you should aim for experience values that will put you close what you would need to evenly level up to max with plat armours and spirits without wasting more xp than needed. You can find the tables for that here. I usually go to the first one that is 50 or higher personally.
I would add to the list of units that benefit greatly from awakening, Saki, who not only attacks faster, but also has her cost reduced by 4.
On the note of using Rainbow spirits, there isn't really a need to wait for SAW if you already know what the SAW is for the unit and plan to use it. Or if as noted the unit already will benefit greatly from skill levels. That said, try to avoid using them when the skill can be readily obtained without the fairies (for instance, if the Big Adventure Anna star event does actually happen soon, you can farm Gadoras, who has the same skill as Despara when CC'd). Additionally, it is most cost effective to use Rainbow spirits on level 1 units, such as just after CCing or awakening since while they do not give experience, their cost still is dependant on the level of the unit. A level 1 unit will cost 200 gold to feed a Rainbow spirit, a level 40 unit will cost 1760. That said, don't be to worried about keeping to that if you already awakened the unit or have a unit with the same skill to feed to it first.