You are getting played here.
he is using your DC to manipulate you. He will say whatever it takes to get his own way. he is actually being abusive to you all.
He is telling you he won't leave the house/consent to a divorce
He is manipulating the children, telling them sob stories, involving them in the breakdown of the marriage to put pressure on you
he abuses drugs and wants you to lie to allow him to continue with them and keep access to his DC.
www.freedomprogramme.co.uk/docs/mrgoodbad-english.pdf
See him in there at all?
Forget trying to negotiate with him, you need a sane, rational, reasonable person to negotiate with. That is NOT him.
Tell him to move out, asap. Restrict all contact to through the solicitor only, refuse contact until he kicks the drugs, and say that you will stipulate that he needs to have drugs tests to see them (cannabis stays in the hair for over a month IIRC)
There is NO hope of an amicable divorce, this is down to him. whatever you do, this guy will kick off unless he is calling all the shots, and even then I am willing to bet he'll start changing the rules and doing what he wants to do.
This man is not one you can ever have an amicable divorce with. Forget that, deal with this as remotely as possible, Get him out of your home as soon as poss and then stick to written communications only.