As someone who has done AP Chem and is doing A-Level Chemistry. A-level Chemistry is not twice as hard, but close to twice as hard as AP Chem, in my personal opinion.
A lot of the content is similar but covered in much more detail at A-Level and the Question Papers are much harder. Questions require a deeper understanding of the same content and the need to explain concepts and apply them to a range of different scenarios is much more demanding in A-Level. I found when doing AP, it was more memorisation and less open-ended, in my opinion.
But if you have done AP Chem it wouldn't be hard to transition into A level (although A level Chem is definitely a two year course, I do think it would be feasible to skip Y1 content with a good foundation in AP Chemistry).
I don't know about ACT tho.