You can go over the cap to re-sign your own guys (as long as you have Bird Rights on him). So I'm assuming BOS can simply sign Kemba using space and then go over the cap to re-sign AL. Haven't looked into BOS situation specifically, but that's my general understanding. Whether that'd put them into lux tax is another issue.
It's more complicated. They have to renounce their rights to Horford and Morris to get far enough under the cap to sign Kemba. Your unsigned players always have cap holds that count against your projected space.
But if they sign and trade Kyrie to the Nets and at the same time Rozier to the Hornets, and the Hornets sign and trade Kemba to Boston, Boston does this all without having to renounce Horford or Morris. It would be huge for Boston. It's good for Charlotte, since they can make a more competitive offer to Rozier.
There are three problems:
1. Rozier may find a richer offer than the Hornets wish to pay (presumably from Phoenix, who are the most logical destination for him)
2. All three teams would be hard-capped and could not enter the luxury tax this year, which isn't that big of a deal in the short term but limits trade options later in the year.
3. The Nets really have to ask themselves what Boston can provide as a sweetener (a first round pick? Maybe more?) that would make it worth their while to help Boston be better than them. The Nets should be trying to sign Horford outright if they don't get Durant, not helping Boston be a great team.
I'd say the first two are pretty manageable issues, but the third is not. I wouldn't help Boston were I the Nets.