Thanks to all for your thoughts, and especially Jacques for really digging into this. Jacques, I checked the Emerson case you cited, and I confess I don't see any discussion of the submit vs. receive issue. Notwithstanding that, it appears that there may not be any direct case authority under 15.505, and the best we can do is conjure on how far other FAR sections may take us by analogy.
For instance, 15.208(a) says: "Offerors are responsible for SUBMITTING proposals....so as to REACH the Government office designated in the solicitation by the time specified in the solicitation." (emphasis added).