Oh, I see you were able to make it to the EX difficulty (fifth stage). That's even better.
Whichever chapter is fine, and to confirm what Volarmis said, what really matters is the difficulty.
CV is the most versatile debuff unit I have seen around. I like how she is able to fill a support role (buffing/debuffing) and as a secondary damage dealer. If you're looking for a solid debuffer in general, I'd recommend her.
Difficulty Rec. Overall Force Medals EX class 580000 x200 Top class / 最上級 380000 x150 High class / 上級 210000 x65 Intermediate class / 中級 129000 x30 Elementary class / 初級 60000 x5
As for Sedum, my issue with her is that aside from lacking a skill activation buff, a lineup with decent attack debuff barely have need for her negate (only good for 1 hit per character anyway). If between Sedum and White Tulip, I prefer White Tulip.
Well, that's the price you pay (aside from the crystals, of course). Whether or not they were fully-specced prior to synthesis, they'll revert to 1/1. It's not for nothing though. If you look at it in another way: You're going to save more crystals to promote a character if she's at 5/4 instead of 1/1, especially if it involves a gacha 5★.
Eligible characters become on par with gacha rainbows after synthesis as their stats, skill effects, and abilities improve to rainbow-tier, so having become actual rainbows, they'll be generally treated like their gacha counterparts. Not a bad trade-off, really. Some of them become absurdly strong, even. Exhibit A: Rainbow Rose.
Lastly, I should leave this here for reference: link
Something crossed my mind recently.
You remember how the discounted gachas (Japanese Anemone's gacha and the rotating theme gacha) last year had the number "3" in their gacha prices? I realized that it may just have been purely in connection with the 3rd anniversary after all. In addition, the special ticket promo during that period rewarded 30 flower stones as well.
That "connection" could explain the absence of such gachas for the rest of the year.
JP devs and their penchant for [playing around with] numbers. ...aaaaand no discounted gacha this time around.