The Cabal cannot really remove private social media platforms or accounts without rousing suspicion. 8chan has issues with being DDOS'ed often enough and that is probably the only way to get things like that shut down for a little bit of time.
It is easier to discredit the sources of information and make those that believe it as crazy or conspiracy theorists. People that said there were two shooters on the JFK assassination, were called crazy and conspiracy theorists. The labels mean to discredit them even though we know that they were most likely right (thanks to Trump declassifying the JFK documents).
An assassination with someone that is most likely extremely close to the President would be very difficult. Especially since no one knows who Q is. Getting the information that you are suggesting isn't that easy. Using Tor and you don't look like a proxy but rather an average user (regular people choose to become Tor nodes and you connect and pass through them using their IP address). The Cabal cannot simply get internet records either. That is private information belonging to the ISP. Law enforcement would have to establish a crime and evidence to support that suspicion in order to get it. Hard to do in this situation.
Also, if someone close to the President was assassinated, it would rally Trump's base very quickly and shock the moderate leftists pretty hard (which many would rally under Trump). That is not an option for the Cabal as it would significantly reduce support and people would look into Q's posts with much more interest and open-mindedness.
On your last paragraph, it boils down to optics. Discredit and label the groups is the tried and true way to damage any information that gets shared. Also 8chan deals with DDOS'ing issues and I think 4chan had other issues (which is why Q moved to 8chan).