For now, we have the following observations:
1. Our target demographic is relatively small, educated, and (incidentally) westernized. The last point is incidental and will likely change, but for now it provides a useful focus, not only by simplifying the design task but making our auxlangs mutually intelligible for the most part.
2. This means we can use Outcome Neutrality as our model, allowing greater ease of learning for the comparatively few who actually need an auxlang (or at least want it enough to learn it).
3. But even Outcome Neutrality doesn't work for everybody. There are Eternaj Komencantoj for every project, and they can't practically be made to learn any one auxlang.
4. However, if multiple auxlangs are allowed, most people can probably find one that works for them.
5. The mutual intelligibility already mentioned means that even if you have trouble with one or two auxlangs, you can probably still understand them. So if you do well in an auxlang the other guy just can't master (and vice versa), you can still each use your preferred auxlang and be understood--and with less overall effort than achieving any real fluency in a natlang.
Thus it's possible to master one or two common auxlangs and just passively understand the rest: a multiple-auxlang solution.
On the other hand, Eternaj Komencantoj will doom the Onelang idea, because although they are a minority, they feed a common meme: Auxlangs Don't Work.
Think about it: we've heard that at Esperanto Kongresoj (or whatever), participants can't communicate and eventually have to use English (French, whatever, but usually English) like everybody else. We have been told that Volapükists couldn't really speak their auxlang either, and that too is a lie.
But Eternaj Komencantoj furnish real anecdotal evidence for the claim, so they are far more important than their numbers justify. If any auxlang does at all well, I guarantee that EKs will appear in the news to debunk it. But if there are several auxlangs, sincere EKs (not those with an axe to grind) can be challenged to try one of the others. That will probably do the trick, and a converted EK can be damning to critics.
All this requires a willingness to consider auxlangs seriously, however, which leads where everyone familiar with my thinking knew it would: the Auxlang Epiphany, also known as the next post.