My DM has looked over my character several times and agrees it's 100% legal. I've exploited a few loopholes, to be sure, but nothing outright rule-breaking. And I consult him every time I level up. If you looked over my character sheet, you'd probably come to the same conclusion. Now, that said, it may not be 100% up to date with various errata, and I will admit to that, but considering how hard it is to find errata, I think I've done a pretty good job of keeping it by the book.

All of what you've suggested here is pretty much on my To-Do list. I'm going to take the Spell Scars Magus Arcana, and I'm also going to start crafting my own PoPs. I think the scabbard is a good idea, too.

I already have a few ioun stones, cracked or flawed (whichever is lesser), which complement his skills and abilities, plus a wayfinder to get those extra properties.

Believe it or not, I have a rationale for being so heavily armed: I can never be disarmed of the cestus, and it doesn't interfere with my casting. My wakizashi are my main weapons, the sai for when I need to do bludgeoning, the crossbow for ranged. The saps will be for non-lethal, and I may switch up to the katana as my main if I can convince my DM to houserule it as a light weapon (because, really, it is), as I already have proficiency in it, courtesy of being a ninja. If not, meh.

As a testament to how effective this character is; in our last session, the party was taking on a tunnel brute and got split up. Me and our squishy investigator (who is practically useless in combat) in one group, and our sohei and ranger in the other. The monk and ranger were outside the church we were in, so it was me and the investigator against the tunnel brute. I pretty much took it on single-handed, and only in the last two rounds of the fight did the other two show up, and only after I had bitch-slapped (literally) him with both chill touch and arcane mark, delivered through my cestus (because the floor had fallen out from under us and we were both hanging by one hand and I had dropped my sword when that happened to cast spells).

That was when Hondo was level 6. A tunnel brute is CR9. That fight got him to level 7. Granted, there was some good rolling on my part, and some not so good rolling on my DMs part, but still.