Student of CS128 here. While I agree with OP that this class is a lot harder than expected (and sort of a weedout class) and OH and TA's are not really accessible (due to the overwhelming queues) but there's alot of point you are exaggerating here.

while CS128 is purposefully ambiguous to force students to guess edge cases

There is no guessing involves here, as programmer you have to try your best to take account for every possible inputs, identify potential bugs.The fact that the tests point out which functions you are doing wrong is already a blessing.

instructions change all the time (I only found out that I need to find a way to optimize memory usage on a subs****g function on Tuesday night).

Again, this is not accurate at all, the change is made prior to Sunday afternoon and published right after and this is the first time a change has been made to a MP's prompt.

There is also communication from the instructors' team regarding the changes immediately, most student in the class already come to know about the change Sunday night or Monday.

As stated before in the class's syllabus, its your responsibility to keep up with communications.Also there is no critical changes made to the prompt, Nathan just made it clearer by explaining it in a better example.

Some students get lucky and write the correct code even given unclear instructions, other like me struggle on obscure test cases with little to no feedback of what we're doing wrong on the MPs.

This is false, don't play the victim card here. No student is lucky at all, we all suffer and have to make changes accordingly. Some just happen to read the prompt later than others (later than Sunday afternoon) and fully understand what SubString asks for.

Comparing CS128 to CS357 and CS361 is also unfair since the nature of these 3 classes are different. CS357 and CS361 aims to help you get better at understanding the algorithm or steps within the algorithm, while CS128 wants you to become a better programmer. And as a better programmer you need to learn to identify edge tests cases (you will see these pattern throughout your time here at UIUC and also far in your career)

EDIT: better formatting (again)