Then get another lawyer.

I already told you third party claim.  A lawyer could represent you against disability, and then bring in former attorney M to have him pay for whatever disability wants from you.  

I don't know why S won't do that.  Maybe your money amount is too little.  Maybe you are a difficult client (suing former attorney is a sign) that he just don't want do the conflict of interest. 

I told you everything I knew. The rest is up to you, get another lawyer if S doesn't give you want you want.