Since the problem is that the threads were created for unnatural reasons in order to overpopulate /s/ with loli, I think forcing it so that theres only one loli thread on /s/ for the time being could work, since the front page would contain adult stuff too then.
Of course the thread could still be "unnaturally" bumped by a single person into the top of the page, but I don't think it'd be pratically different from a natural loli thread by someone who just has a ton of pictures so I don't think it'd be a problem.

Also I think it'd improve discussion if each of the three issues (legality issue, spam issue and scaring away users issue) were discussed in separate paragraphs (or bigger partitions of a post if each argument requires multiple paragraphs). I think trying to discuss all three issues at the same time without clear separation clouds judgement.