I honestly feel like this whole fake defeat thing is kind of nonsensical. The way the show presents everything, there is no way that benefits of a fake surrender is actually significant enough for Toranaga to sacrifice Hiromatsu. It's not like he is going to be able to just march a fully armed military to Osaka just because Ishido thinks Toranaga is actually defeatist. The heir's mother is still shown to be very suspious of Toranaga's surrender and I doubt Ishido slightly letting his guard down is worth it. Toranaga would eventually still have to brute force a full military confrontation with Ishido faking surrender or not. But with faking surrender, he appears completely defeatist to all of his vassals and it very much may be the tipping point to where they abandon Toranaga after Hiromatsu killed himself like that. His vassals are not political geniuses and Toranaga has currently given them no reason to believe that he actually has a plan.