You can't move Chabot if Chychrun is not going to extend. The only exception would be if you think Chabot is an albatross, and by moving him you're getting ahead of the rest of the NHL.
If we want to bring back Karlsson, and Pittsburgh is open to moving him, the trade is something like Korpisalo+BOS 1st+OTT 2nd as a base.
When Pittsburgh traded for Karlsson, they dumped 15.5M in bad cap with Granlund (5M x 2Y) and Rutta (2.75M x 2Y). Korpisalo is owed 16M. So it's effectively a wash cap wise.
They also gave up what is looking to be a top 16 pick, and a 2025 2nd. Given that Ottawa's 2nd is projecting to be in the top 40, a late 1st and a top 40 pick are probably close enough in value to a mid 1st and a 2nd a year from now.
This assumes that they move Chychrun in a different trade and recoup the 1st+2nd that they give up here. So it would basically be 1 year of Chychrun (since he won't re-sign) for 3 years of Karlsson. Karlsson's inflated cap hit would be subsidized by dumping Korpisalo.
There isn't going to be a trade market for Karlsson due to his hit, age, term, and NMC. San Jose had a difficult enough time moving him after a legendary season. So if Pittsburgh was at a point where they were open to taking a mulligan, and Ottawa offered to effectively undo the trade for them, you'd have to think they would consider it.