Ok, I need to get this out of my shoulders...
The idea that Gykamaru/Onimaru wants to boost the scabbards moral is an absolute blasphemy. First of all, let's look at the scabbards in this chapter...
Do they look to you demoralized, at least a little bit? The scabbards have no lack of motivation at all. Let me reapet this for you couple of times: the scabbards have no lack of motivation–the scabbards have no lack of motivation. As matter of fact, they are overly–motivated, so much so, that dispite the fact that they haven't fully recovered yet they still wanna fight back. The scabbards came here ready do die, every single one of them is more then enough ready to die for this war and for Wano country. Again guys, the scabbards have no lack of motivation, at all.
Gykamaru (from what we've seen) is very respectful when it comes to Wano's history, swords, Samurais etc. For him to turn into Oden and/or pretend to be him is a huge disrespect to not just to the Wano pepole but mostly to the scabbards themselves. The scabbards are grieving for Oden and Gykamaru pretending to be him is just pouring salt into their wounds. The memories of Oden will rather demoralize them instead of boosting their moral, it doesn't make sense, it simply doesn't.
Now let's look at what Kawamatsu said;
Kawamatsu has been taking care of Hyori for quite some time, he knows how Hyori looks like. Kawamatsu has also met Hyori with Zoro so he knows how adult Hyori looks like as well. Kawamatsu says "there's no way it was real" but why? Why would Kawamatsu say something like this about Hyori that's alive and healthy, just because Hyori isn't in onigashima shouldn't supris Kawamatsu that much since Hyori could have just snuck in at some point. The words "there's no way" means or strongly hints that that person is ether dead or isn't in Wano at all. Oda spiceficly chose Kawamatsu for this scene since he's the one who met adult Hyori and knows her look, he can easily recognize her.
Conclusion: when it comes to logic Gykamaru doesn't make sense, at all. But unfortunately we've seen that Oda will cross any lines just to surprise the audience. He will lie, he will write one plot into completely different plot whiteout any foreshadowings or hints, all just for sake of doing something that we don't expect. So no matter what I say everything is possible in Oda's one piece.