Thank you for the question
May I ask - why are YOU expected to pay his rent and allowance?
You are entitled to write back that your maintenance payments are meant to cover all your sons expenses other than any "extras" you chose to pay for.
On that basis she needs to use the maintenance to pay for his rent and allowance - and it is for you to "top that up" directly with your son if you decide to do so.
The choice is the hers.
You can pay your son directly - and then you are both responsible for your young adult child - or you can pay your maintenance to your ex and she can sort his finances out.
That is the approach that the court will take.
to answer your specific questions
2. It does not say so but it is implied
3. Unlikely - and approaching CMS at this point would not be wise
Highly unlikely - and the payments would then be made payable to your son!
You are most welcome I hope all goes well