I think you're making an assumption here that may not be valid. There aren't very many specific rules about how to handle attributes for a monster that begins play with an acquired template, so it isn't clear to me that you have to treat an acquired template as a post-pointbuy sort of thing: I think it's just as valid to assume that the post-template score will be treated as a racial modifier to apply to your point buy. Naturally, your assumption is reasonable, but I don't think it's secure enough to call it definitive RAW.
Also, you've got to be really careful on how you let this kind of thing influence your LA votes. An Intelligence score of 1 completely invalidates a playability rating, in principle. That's why the thread guidelines explicitly tell us to assume that some means of acquiring a playable Intelligence score is available. Granted, that doesn't mean we can just completely hand-wave away the Intelligence problem with this template, but it does mean that "tanking your mental stats" shouldn't be treated as a very reliable counterbalance to whatever boosts and boons the template gives.
To be fair, Zombie is a template that adds hit dice to the monster, so even at LA -0, there's still a cost for taking the template. Topiary Guardian is an oddball template that creates a whole new creature with hit dice and ability scores that are set and completely unrelated to the base creature. And skeleton was given an LA that varies with the base creature's hit dice, which is something we really haven't done since then. Yellow musk zombie is a strange template that takes away a lot of character agency, and even saddles the monster with a short lifespan of just two months +1d4 days.
I don't disagree that it's possible for a template to get LA +0 or -0, but I think those are all very unusual circumstances, and I don't think necromental amounts to a similarly unusual circumstance. And I don't see this as a legitimately zero-sum template, so I think it has to have a positive LA. I'm voting LA +1* for the necromental template.
I also agree with you that the create spawn ability is inherently limited by its narrow niche; but in principle, it's uncapped, so I still think the asterisk is justified.