In our experience, the challenge questions in the AoPS text (second half of end-os-chapter problems) tend to offer a lot more complexity than Alcumus. Even in topics he thinks he has, you can jump to the challenge Qs and see if he's still flying through, or actually needs to stop and think. If the former, jump to Algebra; if the latter, savour the pre-Algebra for the complex problem-solving it teaches.