Hold on now. That Kemba trade shaved ~$7 million off next year's salary. If we did a sign and trade for Ball at say around $19 million while sending out Smart and Thompson(one or both could be redirected elsewhere), that could take another +$4 million off the salary. The team at that point is:
Ball, Brown, Nesmith, Tatum, Horford
Pritchard, Langford, Parker, Grant Williams, Timelord, Edwards, Mo Brown
That's 12 signed guys, a team salary around $123 million and the apron is going to be a bit above $142 million.
If Fournier will sign for $16 million first year, that would still leave $3 million or so to grab a vet and the team could go with 14 players and sign two 2-way players who do count against team salary.
My math might not be 100% accurate but I think a sign and trade for Ball might work. Yes, the team would lose a trade exception or two but
Ball/Pritchard/Vet min guy
Fournier/Langford/Edwards
Brown/Nesmith
Tatum/Grant/Parker
Horford/Timelord/MoBrown
Could be under the apron and looks like an improved team.
If others want to run numbers, it might be doable. I think.