Your initial question and title was about Avios.
I think that you got that extensively answered.
If you want the avios, use your BA. Whatever experiences you got on flying BA is irrelevant to that question as you won't be flying BA just crediting avios.

If you feel that the 24/48 hour rule is more important then use your new CX account although you will get no avios.
Personally, I can always check-in 48h before on CX, but it may be because I have status on BA FFP.