You are right in principle. The question is whether Portland is going to desperately hold out to dump their more expensive contracts, or if they are going to concede dumping their halfway-useful to useful contracts.
First off, this doesn't quite work under the cap rules. However, we can throw in Langston Galloway and make it work. That's a win for us anyway. Assuming Galloway doesn't opt out, Portland only saves about $12.5M in salary, but that gets them back to the tax level, so it's a big savings.
For us, we pick up about $13M/$21M/$22M for those three years, which seems kind of steep for Harkless/Leonard unless Harkless really pans out. If Portland looks to make that deal, I might hold out for #15 and some other goodies. But it saves Portland something like $55M AND luxury tax on top of that, potentially for three years depending on what other adds/drops they can make. Let's put it this way - if they don't get rid of anybody, they're in the tax three straight years guaranteed, and that's just the players they have now plus min salary guys to fill out the roster. Can they afford to pay the repeater tax for the 8 seed every year? So it's not inconceivable that we might be able to, say, get their 2019 pick to replace the one we sent to the Sixers on top of #15.
Depending on what we can get Portland to give back, I'd be tempted to do that. But I'd really want #15 AND their 2019. It all depends on how desperate they are. I don't think anybody wants to take Turner off their hands. And outside of maybe the Nets, who signed Crabbe to that silly offer sheet last year, I find it hard to imagine anybody will go for Crabbe either. So Portland may in fact be motivated to send off Harkless/Leonard. We'll see!