I think the sponsor must first address the second and third "If..." sentences in Post #2, in light of what was intended. My reference to "prospective" is that the sponsor should then amend the plan to incorporate the intent, for any future new hires.
A self directed DB plan - not a good idea. You can give market rate earnings in a CB plan, but considering the minimums required, floors on the hypothetical account balances and such stuff, not a good idea even if legal.
1 is too few, 13 million is too many. So somewhere in between. Like jpod I don't thing there i any direct guidance on the optimal number of investments. It is left to the prudence of the Trustee's to decide.