In the corporate world at least, there is no 'gardening leave' clause in any contract based on my experience. All your employer does is pay you and tell you not to come into the office. As an employee, you're getting all the benefits of the contract, but not the obligations.
I cant see that it would be any different in the football world however if we're not allowing him to earn contractual benefits (ie by firing him before he could earn them), then he could claim that he's being deprived of the opportunity to earn them. That assumes that the bonuses are 400% contracted, which I'd imagine they are. Once we mathemetically cant qualify for the CL, then those bonuses for this season at leats would fall away.
I think Mourinho will therefore be around for another say 4-5 games or so, depending on results, and then fired. The atmosphere at the CC final could be interesting!!