The decision about S11 is between you and W, but of course S11 has a say.
If S11 is sleeping on a sofa with interruptions plus a crying baby and it is interfering with S11 progress especially at school, then the best is for him to stay with you in a stable peaceful environment. But HP and W decide ultimately about S11 schedule. I really don't understand the sleeping arrangements at W aunt, they seem lacking in clarity.
I think this is one where you will have to discuss with W a revised schedule if the night does not go well. Besides shouldn't W be on the sofa and S11 in the bed room or in a cot bed in Ws room? W hasn't handled this well at all in my opinion, but perhaps when she is a more stable home environment this will improve. It is very early days and nothing needs to be settled in stone. Sounds like temporarily S11 may need to be with you HP.
You handled your move to the Condo well and settled S11 into his new life there with great grace and planning.
The car, 30 days is generous and an extra week more than generous. It has achieved what you wanted it to achieve peace and less hassle over Xmas and the great move but time for W to manage her own transport needs now she is at aunts. However I think I would tackle this car issue in a different conversation to that of S11 schedule.
Stay calm, you can do this with kindness, after all you both want the very best for S11 and his stability and wellbeing is paramount. This is another transition event HP, you can meet this next challenge.
Peace of mind and a alcohol free time Vanilla
Freedom is just another word for nothing left to loose. V 64, WAW