I also agree with #1-3 and the qualifiers that taking the AP test for an IB class can be useful. (We’ll find out, as my son plans to take the AP Spanish test after taking the IB Spanish SL class this year.)
At our school, some students are advised by the GCs not to bother with the corresponding AP test for dual-enrollment classes offered on the HS campus. They may be unaware that the dual-enrollment credits work fine at any California public, but don’t transfer to privates. At the time they take the dual-enrollment class, they may not have thought about privates or OOS colleges.
Regarding #4, self-study can also be of use in particular subjects where an AP score could permit more advanced coursework during high school.