For that much debated final roster spot, the Celtics could convert a two-way, sign a G-Leaguer, or pick up a free agent vet. Two way players can be converted anytime up to the last day of the season and I assume be playoff eligible. Davison appears to be a worthy player but we don't really need another PG. We don't really need anything but a big or a wing would seem to address "thinner" positions.
Of the G-League wings, Shayok, Valentine, and Snell, Shayok has actaully been playing the best, at least based on box scores. Snell and Valentine have actual NBA experience though. Snell at 6'-6" has the size you would like. We could of course also pick up someone from another team like Utah did when they grabbed Samanic.
Converting Kabengele makes some sense too but I just don't think he is that good. I can't see him playing a single meaningful minute in the playoffs. I guess that is true for all of these guys.
As to other NBA vets, I would think if we liked someone, we would have already signed them. I can't see us bring in someone at this point. The 2-way'ers have been with the team all season so kind of know the drill. The other Celtics G-Leaguers at least should know the basic systems or schemes.