I wonder if it will come down to Pageau Da Costa Hoffman and Lazar fighting over 1-2 spots in camp. I can see pageaus defensive game giving him a slight lead over the others, unless the Spezza trade brings back only prospects, than Da Costa will almost certainly be up.
Those guys will mostly be competing with different players, although Pageau and Da Costa will likely be competing for a center spot. Hoffman plays the left side. Da Costa is a center, and only center. Pageau plays center, but can slide to the right side. Lazar is center / right winger, but likely to start at RW, if he can crack the Sens next year (a lot more responsibility on the center, not to mention getting used to the draws against pros). And yeah, the 9 game audition will probably make sense regardless of how he looks in camp. Get him some first hand experience, even if you are leaning towards sending him back before the season starts.
So Hoffman will be competing on the left side, where MacArthur and Greening are slotted in. Prince and Puempel will also be vying for a spot. Also likely a UFA at LW, if Michalek isn't back. Condra can slide over the left side, although typically plays RW.
At center, if Spezza is here, it will be very crowded. Spezza, Turris, Smith and Zibanejad are assured spots, although Zib might slide to the right side to make room for Pageau or Da Costa on the 4th line.
On the right side though, it is equally crowded, especially if Zibanejad goes right side. You have Ryan and Stone as well, along with Neil and Condra. Tough for Lazar to fit in there, unless Zib is center, and even still, he has to be better than Condra overall to make the team.
The real swing event is if Spezza gets moved, because everything then shifts around, and is somewhat dependent on what is coming back in the deal at forward. Better chance for Lazar though probably. Or maybe not.