IMHO, and taking into account the rest of the information, nothing. Basically, kanly is the form for the limited, very often multi-generational vendettas between the Great Houses. It’s to keep the collateral damages from such fighting limited, especially in light of the generational conflict. Fundamentally, the Baron is offering to settle, despite the fact that (from hints in the OG and explicity in the prequels) having arranged for Leto’s father’s death, not to mention the various acts against all the top Atreides advisors (Gurney and Duncan especially) prior to their joining the house.
It serves Leto poorly to just agree to let “bygones be bygones” and if anything, arguably would have made things worse, as that weakness would destroy the partnerships he had built in the Landsraad. Fundamentally, the Baron is saying “Make peace or we’re going to destroy you!” and Leto thinks they’re serious, but that he’s sufficiently prepared to turn it around.
The fact that the Emperor is helping ensure the fix, and that the traitor is in place means that the Baron has no real reason to settle. He’s just playing the game, as he always does. And if Leto had caved, he would likely have had no problems finding a new justification, demanded some payment that Leto could not have met and it proceeds with minimal delay and a weaker position for Leto. Being the Baron, he would likely have demanded Paul be sent to him for fostering, or Jessica as tribute (after all, she’s not a legal wife nor a noble).
No matter what, Yueh was an almost unbreakable cheat for the conflict - even in a more settled situation, the unbroken trust would have left him with access to the shield generators and the Ducal family. More time might have left the battle ever so slightly less one-sided, but the results were set with Yueh and the sadukar.
Granted, the wildcard would have been the Fremen, but no one knew the numbers and prowess of that factor, and it is extremely unlikely the Duke could have earned their full confidence in any timeframe the Baron would have allowed.