DONT WASTE YOUR TIME READING THIS
Let's assume for a moment Hayward for Turner + McDermott (as a basis) goes through. Then the Celtics have:
Guards: Walker, Smart, Pritchard, Edwards + Waters (two-way)
Wings: Tatum, Brown, Nesmith, McDermott, Langford, Ojeleye
Bigs: Turner, Theis, Rob Williams, Grant Williams + free agent? + Fall (two-way)
Walker, Brown, Tatum and Turner are basically guaranteed starters then. The options to start alongside them are Smart, Nesmith and Theis. As a closing line-up I obviously choose Smart, but to start him makes rotations really wonky. Pritchard would need to deliver immediately and given Langford's injury, you'd probably end up needing Waters/Edwards to play some. The option to add a pure shooter in Nesmith makes sense, you'll also be able to give McDerrmott an important role on the bench. It will squeeze the minutes of Theis and Rob Williams though and it will force Brown and Tatum to defend bigger players.
When I see that roster above I see the following players being ready to play on a consistent acceptable level (G: Walker + Smart yes, Pritchard maybe; W: Tatum + Brown yes, Nesmith maybe and Langford - injured; B: Turner + Theis yes, Rob and Grant Williams maybe).
Thus guards 2(+1), wings 2(+1) and bigs 2(+2). This gives me 10 players for the regular rotation. To balance these minutes a 1/2 guards + 1/2 wings + 2 bigs line-up as basics makes the most sense. Luckily we have two of those bigs that are good floor spacers (Turner and G-Will). Just fill in the pieces and take into account that Walker/Smart/Brown/Tatum/Turner is the closing line-up, we end up with these rotation minutes:
Walker (30) / Pritchard (12) / Edwards / Waters
Brown (33) / Smart (30) / Langford
Tatum (33) / Nesmith (18) / McDermott
Turner (30) / G.Williams (15) / Ojeleye
Theis (21) / R.Williams (18) / vet min / Fall
It would need to be an adjustment by Stevens to regurly play two bigs, but I think that most combinations of Turner/G-Will/Theis/Timelord are playable. With the roster balance in mind I understand why Ainge is pushing for more than Turner. If TJ Warren could be obtained (in exchange for Theis) it would change things a bit. Then you could play Warren/Turner as a starting frontcourt with Grant and Robert Williams as back-ups.
And in this scenario the Celtics can still dug the tax even by taking on both Warren 12M and Turner 18M. Maybe Ainge has had this in mind all this time, because all the cost-cutting moves make sense then as well as not being worried about signing major free agents. (Walker+Brown+Turner+Smart+Warren+Tatum+Langford+Nesmith+G.Williams+Pritchard+R.Williams+ Ojeleye+Edwards+Waters(minimum)+vet min < 132.6M). Those starters + Smart would earn 84% of the total payroll, would be some impressive cap/money management!