Supposedly, the first couple of events will be repeated once in a while. Last I heard, is that she should return in about 5-6 months now. She deals a lot of damage, and is one of very few girls whos special attack takes two slices from a Wealthy Fly instead of only 1 (another girl who has that is Sasanqua, the yellow 3* girl, and I have heard there are a few others). But, I really can't say that she's a must-have, unless you're short on hit-girls anyway.
Edelweiss is very nice if you have her with full skill and equip but to be honest I find Toad Lilly to still be stronger in more situations. You cannot always get the nests to gain that 10% hp but you do always hit pests and her defensive stats are actually equal or better then most 6* girls. Have to admit tho for 12-5 easy mode clears, having both of them and both on a helper squad is amazing!
Oh and if you missed Edelweiss then you also missed Lavender and to be honest she is one of the stronger magic types in the game right now too. Thankfully for those who missed her she will likely be revived 1st as she was the 1st event girl.