I'm not expecting real answers here, just thinking aloud:
So what about preview pages published on review sites which can be up to eight pages of a regular comic? What about comics that are legally published in full and for free on such sites?
If the preview pages take us to the 1/3 limit does that mean no one else can post any more of it when it's actually published? Does that mean, if a long preview was posted the previous week, we'll have to discuss current comics around alternate scans?
How far back into the archives must a poster look to determine that they are not exceeding the 1/3 limit in a new post? How much more of a comic can be posted after that period? I presume the argument "it wasn't tagged properly so I didn't see those pages from 2 months ago" will be a poor legal position to take and will certainly not help us avoid a takedown notice - so how much effort most a poster put into searching the archives for other scans before posting their own? What if the older post is edited to remove some of the scans and "make some room" for new ones? [yes, that last one sounds dodgy to me too]
If it is decided that older comics can have more pages posted that leaves a lot of room for interpretation: how old, how much more? Are we going to end up with some sort of chart: time vs. % of pages? Will this actually leave s_d open to further legal challenges by apparently suggesting an entirely fictional copyright term for comics? Normally a work cannot be published in any substantive form without the rights-holder's permission until 50-95 years after the death of the principal creators. Saying that a complete comic can be posted 50 years after it is published could well be technically (and legally) false (no matter how ridiculous we find the idea).
Could we get into trouble for posting a single page from a new comic that shows a significant revelation, one could be described as "the heart" of the work? That is, could posting the climax of an arc alone be argued as harming the market for that work by giving away the ending (something else that copyright technically protects)? [actually this one is kinda scary]
How do we stand on posts linking to other sites (or other journals) where a complete story is posted?