I doubt Tsubaki is in super immediate danger, and there's a reason for that: Firstly, It looks like Suzune hasn't contracted as of Nadia's last visit, and the impression I get is that they were together for some time. Not years, but likely months. Plus, from the look of things it seems Tsubaki only witched while she was grieving for an old, close relative on top of sharing her seeds with Suzune.
If Nadia has already met Tsubaki on this round of her travel's through Japan, then we can be pretty sure we have time. Might be worth asking Firn about that.
------
Related, of all the PMSM girls, Suzune, the Hinata sisters, and Kanade Haruka are the ones most at risk as they all seem to contract young-ish, with Chisato being a bit more ambiguous as far as the date of contract goes. Assuming Tsubaki is saved. Kanade is the next most important to intercept, as her wish, made while she thought she was dreaming due to not believing in magic, erased her sister from existence.
I kinnnnda want to prevent that.
I really don't think a fly-by cleansing is likely to be on the table. Any meetup in person with someone we haven't met in person will likely involve a longer talk. I'm willing to drop a lot for that if it's truly urgent, but it is a time constant, and it will break plans.