Nicely written
SW4I must add that the assumption questions can be handled in below way too -
Step 1: Identify the conclusion of the argument, segregating it from its premises.
Step 2: Eliminate options that are out rightly out of scope, or talks in opposite direction of the conclusion (in good cases, you can remove up to 3 options)
Step 3: Apply negation test and see if the negated statement
breaksthe conclusion of the argument. If it does, you have picked on the right answer.
A common pattern that I have observed includes "unrepresentative sample" as well. The argument gives you a situation, and on the basis of it concludes something. The answer option when asked to strengthen/weaken the case can be done by showing the sample is representative/ non-representative. There are a lot of questions in OG, and Verbal Supplement that tests it.