You do not need to excuse students from activities they are never assigned to. When you create the replacement activities, the due date "assign to" defaults to everyone (or "everyone else" if you add a second due date for the alternate students). You can delete "everyone" or "everyone else" and only assign to the selected students who need it. For the original assignment it is faster to only excuse the one or two students who do NOT need it, but for the alternative version it is much faster to only assign it to those who DO need it.
For activities you do not assign to students you will see greyed out boxes in the gradebook that do not accept a grade. The students (and observers) will ONLY see what they are assigned to. The specific students who are excused for the purpose of an alternate assignment (and their observers) will already be aware of the accommodation, so hopefully this limits the potential for confusion. I am not suggesting any change to that.
I agree with you about mastery paths, which is why I do not have enough experience to do more than offer the page from the guide. Those dedicated to make it work, seem to like it, but for me the lack of a preset due date is a deal breaker.
As an instructor using canvas to teach classes, I do not know precisely how canvas is programmed. If sequential order does not matter for your module activities, and the module in question is not a prerequisite for another module, you do not need to test this. You can make every single activity in a module a requirement, and students will only see the activities/requirements that they are assigned to. The individual activities will be checked off as students complete them. (The real answer will be revealed in whether or not the modules are marked completed after students do only the activities assigned to them, but again that check-mark has no consequence in this case.)
Otherwise, all can offer in response to your question is my own experience, because I DO require sequential order. Out of the same concerns that you had with module requirements, I assigned the activity that needed alternates as the last activity in the module. The main activity was a requirement, but the alternate was simply listed immediately before that requirement in the module without any programmed requirement. Students who were not assigned to the alternate, never saw it. Since it was not a module requirement, they were also not slowed down by it. The lack of a checkmark on excused activities was not a serious problem for the few students that needed an alternate, because the modules are independent of each other. Therefore, I did not pursue the issue further. (Since you are actually individually assigning 150 students to the original activity, you do not even need to worry about the "excused" activities. Students will not even see that there is a requirement they are not assigned to, and so they will not be aware that they should look for a check-mark.)
It is possible that I was over cautious. It is possible that both versions can be requirements and no students will be held to a requirement they are not assigned to for either sequential order or as a module prerequisite. I did not need to test that. If you do need to test it, you may need to ask your admin for a sandbox/workroom and a couple of enrolled "fake" students. Then you can test the module requirements to see exactly what works and does not.
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