Per who DA's top targets are things are clearer:
//The Celtics have a ton of cash and are geared up to use it. Target number one is Kevin Love. Target number two is Greg Monroe. Sources close to the process say the Celtics plan to swing for the proverbial fences this summer including runs at Gasol, Leonard and Butler too.//
http://probasketballtalk.nbcsports.com/2015/03/03/report-celtics-top-free-agent-target-is-kevin-love-greg-monroe-no-2/
A 24 yo 6'11' double double monster like Monroe would make me super happy. (Should I add that Danny is right to prefer him to Jordan
)
Bradley, Thomas and Wallace eat a lot into our cap. Bringing back Bass and Crowder eats into it too. Will be interesting to see if Ainge trades Bradley/Thomas on draft day for picks and cap space.... free up the books so he can make a run at multiple free agents.
You're understating how much cap room. You mention bringing Bass though, so by your estimation Ainge would prefer to clear space by trading Thomas and Bradley, who are both better and more useful players than Bass, than by simply letting Bass go? Doesn't make sense.
As for Crowder, it's really all about timing. He could eat negligible amount to plenty, depends if he signs an offer sheet or not.
Wallace can be easily stretched to cut down his salary impact plenty, if not out right traded with a 1st round pick if there are any takers.
Making a run at multiple free agents it's not dependent on letting go of Thomas, Bradley, etc.
We have 26 mil in cap space before adding Crowder, Jerebko, Bass, #16 and #28.
So maybe enough to sign 1 max contract guy. Stretching Wallace would add another 7 mil in cap space, but if the plan is to go after multiple stars, we might need to make some sacrifices. Wouldn't signing Kevin Love alone cost us 20+ mil in cap space?
Yup, that's right, we only have ca 27 mill cap space and we almost certainly will resign Crowder.
Also, I do not think Danny would like to commit the whole of our cap space unless we become instant contenders and IM Humble O adding Love and Jordan will not make us contenders (the Clips had better players around Deandre and they are unlikely to go through SAS).
The BIG Free Agency comes in the summer of 2016, so I'd save some ammunition for then 
My point is that even if we stretch Wallace, we don't have enough money to sign both Kevin Love and DeAndre Jordon. Not enough cap room. After our draft picks (and Crowder at the very least), we're looking at like 20 mil in cap space. Stretching Wallace pushes us to 27 mil. Love eats up 20 mil. You aren't signing DeAndre Jordan for 7 million dollars.
This can be resolved by trading Wallace + a pick to a team who will eat his salary and then trading Bradley for a draft pick. Then maybe you have enough to go after two top tier free agents at once.
Renounce all free agents. Do a pure salary dump of Wallace and Turner, taking no contracts back. Trade any remaining 2015 first round picks for future picks. Cut Pressey and Babb before they become guaranteed. That leaves 8 players under contract for around $27 million. If you move someone like Zeller for no money in return, you can probably account for the cap hold for roster charges and sign two players at around $20 million each.
You could also do something like send out Wallace, Turner, Zeller/Olynyk/Sullinger, and picks as part of a sign-and-trade for a major free agent. That might allow you to make moves without renouncing Jae Crowder and his small cap hold, then using his Bird rights to reward him.
Its doable without trading Bradley or Thomas. It's not even difficult to sign two major free agents if you have a team willing to do a sign-and-trade.