Engagement Swashbuckler tanks are pretty much just Defender Stance + Persistent Distraction. You pick up a lot of enemies and just passively debuff them.
For Eder specifically, I'd look at something like this:
I've built Eder this way a few times and mostly let the AI handle him. If you keep him at the front of your party and mostly try initiate combat by un-stealthing him, he'll naturally pick up most of the enemy frontline. I generally have a sturdy melee DPS/debuffer up front with him and then 3 squishies, so I can confidently say that this set-up covers the tanking role fairly well.
The gear recommendations are way more "suggestion" than "requirement". One of the nice things about specifically Eder as an engagement tank is that his decent MIG/CON paired with Constant Recovery make him reasonably sturdy even naked. If you just give him whatever is the most defensive loot you have on hand he'll do fine.
If you're using a mod that changes Eder to Unbroken/Streetfighter most of the advice stays the same. Personally I'd turn the mod off, since it takes more micro than I like on my tanks to maintain that flanked/bloodied status. The gear advice would probably change to Reckless Brigandine or One Dozen Stood as armor options and you should definitely go Weapon and Shield Style but with Last Word and a shield, using a slashing weapon as the main hand for your second weapon set. I'd recommend Marux Amanth again but there're a ton of great sabres and half-swords. Absolutely do not take the Flanked immunity weapons. Same advice applies to the Iron Hammer build (now that I look at it, it's basically the same as my suggestion... should've checked earlier and saved myself the typing).