It's not the norm on this forum to give specific dosage advice as this site is not a replacement for vet care. I would suggest you ask your vet, as they know your bunny's issues and medication interactions. Just because something is herbal does not mean it won't have side effects. You can do your own research on dosage and side effects - in humans echinacea can cause stomach upsets so I would be careful introducing this to your rabbit.
If your vet does not advise you on dosage, you could get into contact with a herbal vet (
https://herbalvets.org.uk/find-a-herbal-vet/) for some help.
I appreciate you're trying all avenues to help your rabbit get better, that's what we all want here! I give my bunnies some herbal remedies but to be honest personally I don't think echinacea will help with your bunny's abscesses more than antibiotics and other vet care would. Even in humans, echinacea
may help shorten a cold if taken at the beginning of symptoms. It sounds like your rabbit already has an infection. If I were you I would ask for a sample of puss to be taken for an antibiogram - this should help find the best antibiotic to target what your rabbit has. I worry that delaying giving more antibiotics may allow this infection to get a worse.
You were sent some good advice on your first post, by people who have helped bunnies through similar problems. If your vet does not want to treat the abscesses further, I would get a second opinion, despite their silver status. The problems your bunny is having sound very serious not to mention painful. Is she on painkillers at the moment?
Sending good vibes for your bunny