Remember, the resolution merely asks the Affirmative to prove the need for a reform. In a sense, the Aff plan is just a possibility that shows that we "should" do (whatever the resolution is.) So are counterplans and minor repairs. The fact that the Aff plan demonstrates a reason to adopt the resolution is not necessarily contingent upon whether the plan is "advocated" or merely "presented".
For my benefit (because I rewrote this post three times before deleting it all and starting over), I'm going to rephrase what you said. Hopefully it'll be accurate-- let me know if it's not.
The aff's goal is to uphold the resolution.
The aff upholds the resolution by proving a need for reform.
The negative's goal is to show the aff has not proved the need for reform.
The negative can show this by presenting hypothetical policy changes with which reform is not necessary.
My primary issue with the aforementioned logic stems from my position on fiat. Basically, advocacy grants fiat. Fiat is created because we want to debate about the actual issues at stake-- not the political possibility of X policy occurring. If there's no advocacy (e.g. if the team is not actually defending the merits of X policy), then there's no reason to grant fiat. If there's no reason to grant fiat, then we shouldn't do so because fiat contorts the world, which is a bad thing unless there is some legitimate reason to do so.
Thus, because a non-advocated hypothetical MR has no fiat, all you're saying is that it's a possibility in the status quo-- not an actuality. The affirmative still has proven a need for change because we can't assume the minor repair would go into effect without fiat, and the neg would need to advocate for it-- not just propose it hypothetically.
(I think I finally explained it properly, the fourth time I rewrote this post... but if it doesn't make sense, let me know and I'll give it another shot.)