I'm not claiming any expertise when it comes to the rules of the salary cap but my lay-man's understanding is;
A player's salary is counted towards the cap value for the club with whom that player is registered, regardless of where the money is coming from. If Russell were to transfer to Salford then his whole salary (presumably with some adjustment because we are 2 months into the season) goes towards the Salford cap, even if we are subsidising them.
The amount that Warrington are paying towards his salary could be classed as income to Salford and therefore could be used to increase their total cap level which is, presumably, below the maximum.
I've no idea whether Russell's move was a necessary part of the Charnley deal but, if we have funds available, I believe that we still need a hooker, ideally someone who could also play at 13. Clark and Westwood are both going well but last season they missed 7 games each due to injury plus Clark missed a further 2 due to suspension and Benny 7. Clark was definitely playing injured towards the back end of the season.