I didn't notice Knotweed +50% damage taken. I would say it's not a big deal once you're at 1% HP but it will apply on your retain HP and that's annoying. I guess you want to focus on evasion anyway.

Keep in mind that Adlay Valentine is an exclusive gacha 6* and won't be available in the regular gacha pool. She may be (really) hard to get in the future. If you like her, take her. That's the most important.
With time, good girls can turn bad (ex. 90% of rainbows and most of promoted girls) and bad girl can turn good (ex. Golden Lace).
It's hard to decide but if you always wait and see, girls will be useless or outdated by the time you get them. I can make a huge list of girls like that. I wanted them, waited a long time before I could get them, never used them and regret no getting them when they were good.