I'll put my money on a 3rd two-way player to bring the roster up to the 14 minimum.
This isn't quite right. The Celtics may well (and should) bring in a 3rd two-way but that player won't count towards the 15 regular roster spots. I find it odd that Harry Giles went to all the trouble of getting the waiver or whatever to be able to sign a two-way contract but so far, he has not signed one (unless I missed it). I don't know how the Celtics could do better for that last 2-way slot and I am not sure there is a better situation for Giles.
As to Champagnie, there was no real reason for the Celtics to waive him now, based on the limited guarantees (I think it was $50,000 coming up due), so I suspect they are doing this to give him a chance to pursue something else, to get in with another team that may have more of a role for him. It was always clear that he was just keeping the roster spot warm anyway. I feel pretty confident that they will sign or trade for another vet. This does not necessarily mean a transaction is imminent, but Champagnie was not likely to make the opening day roster in any case.