I believe, and some folks here know the cap better than I do these days, there's some options for Celts...though not cheap ones.
For example, they could trade a pick to the team Kyrie wants to sign with to make it a sign & trade, and create a trade exception. They could then trade for a player into that exception (which could, in theory, be Kemba via a separate sign and trade, or anyone with a big contract). The cost here is likely high because the signing team has no incentive to play along except the value of hte pick they get. But it's an option, isn't it?