How to turn off combat signals

So, there are two ways that you can complete this quest without wrongly accusing somebody like Kalkstien, Rammsmeat or Vincent.

To do this, you need to either successfully complete the Autopsy, by expressing extreme suspicion, and having previously read the books Forensic Medicine and Zerrikanian Insects and Other Vermin, or complete the quest A Gravedigger's Gratitude and explore the catacombs in the city cemetery. Since this can't be done until you clear Vincent, that'd appear to be why you're dead-ending.

To clear Vincent, either don't be a dick to him, and he'll remove himself from the list of suspects through normal dialog, or complete the quest The Rat, then speak to Coleman on the docks. (Note: if you let the Scoia'tel in Chapter 1 buy weapons and go, they will kill Coleman and break the The Rat questchain.

Your final option is to make the false accusation, and do what Raymond tells you.

Yes, he's leading you on and shifty as hell, but you've missed the opportunity to prove that. So dance his dance and eventually, he'll come clean. You miss out on some nice XP this way, but it doesn't break the game or have any real lasting outcome in terms of the narrative as far as I know.

They are sensitive to fire, using Igni to ignite them works pretty well. You can use Argentia oil on your silver sword if you have the material to increase your damage.

Be careful to never engage more than one of them, they can't move so you just need to position yourself well. There is also a named archespore in the swamp (Coccacidium) which can easily kill you if you're careless. It looks very similar to an echinops and can spawn in several locations. You'll get a trophy for killing it. You might have to avoid that one until you leveled a bit if you're having problems, but you should be able to kill the normal echinops.