How can I steal a duchy under my liege?

Ok, I've started as count in the middle of Germany in 1066. I was trying to get myself the only neighboring county in the same superduchy, alas my idiot Chancellor (16 stat) failed for decades to fabricate a claim on it. Eventually the superduke became a king and created a duchy with appointed figure in de-jure borders, and now I'm part of it.

So I can't go after most of my neighbors since they are now part of the different kingdom. I've tried to get some land from other duchy of my kingdom since darn new duke owned me a favour so I pushed him to war on my claims, but then darn king (previously superduke) intervened and declared realm peace. Lost a fortune on mercenaries because of the crowned dumbass.

What do we have:

• Duchy with 3 counties (I own 1)

• Have fabricated claims on both other counties (but not on a duchy)

• Not a slightest idea how to move up in a world.

If I go after duke's county, I also have to fight with the other guy as well (warscore limit is much increased) so I'll probably become bankrupt before I enforce a peace. I'll also lose my coat of arms since I'm basically revolting against my liege. But even if I don't become bankrupt and king won't intervene once more, I suspect I won't become Duke since.. well.. there is already one duke. I can't declare my self duke just because I have 2/3 of existing dukedom.

...and I know this because I tried to go against another count (bishop-prince or whatever) and all I can become is a count with one normal county and one useless bishophood or whatever it is. And since I cannot advance in technology while I'm count and switch inheritance law to something more primary heir orientated there basically no point in taking bishophood while I'm remaining just a lowly count anyway.

So any advice how to grab myself a duchy?

Update

Ok, it's really frustrating. I don't understand how it works anymore. I tried to replay the whole thing and it became even more confusing.

First of all, if someone want to give it a try: I'm playing with all major DLCs as Count Saarbrüken of HRE in 1066. My liege (initially) Duke Fecalhead of Lower Lorraine (didn't lasted for long), Duke Fecalhead the Second (his son), and Duke 'how in the Hell I became one' of Metz (of newly created duchy of Trier).

My county is de jure part of duchy of Trier, but in begining of the game it does not exists and it's territory is split between Lower and Upper Lorraine.

In my new attempt I was like really nice guy. I've gifted to Fecalhead the Second a little something while he still was an heir, really tried to be on the good side of the Emperor since I've noticed it's him who creates new titles (offered aid and took part in few military campaigns of his, got his opinion of me over 80!).

The whole time I could not fabricate a single claim on anything! I've read something about that my Chancellor (12 dip) can be bribed by enemy party. So I improved his opinion of me somewhat and pushed him to lands all around my county every year or so - no result for 10 years.

Eventually a war broke out between HRE and northeren pagans of Pomerania. It was too much for HRE (despite the fact that I killed the guy who was the original nemesis of the Emperor in duel). Pagans gave a few beatings to emperor's army and amassed a stack of 16k troops while HRE left with something just over 3k.

Also there was some revolt for independance going in one of Upper Lorraine counties.

'My chance' - I thought. So I've declared independence of Fecalhead the Second. At first this little son of a Fecalhead the First conscripted his own levies and of his vassals. 'Ok, so there is a guy who was hiding behind our backs while we're fighting pagan barbarians'. I've hired mercs. 'Bring it on'!

Then suddenly: Your Casus Belli is no longer valid. War is ended in white peace. WTF? I see.. Now I'm no longer a vassal of Duke Fecalhead the Second, I'm a vassal of noname guy of Metz who became Duke of Trier suddenly, despite the fact that Metz was part of different de facto duchy.

So in one second Duke Fecalhead the Second without military actions became a king of an entire area, reshaped two existing duchies creating third one and installed a new Duke who was basically just a small fry as I am as my liege. And yes, I took a bath on buying mercs once again.

So it is how revolts go? 'We are fighting for our freedom!' - 'Oh, you see, we've sold you to the other master. Your casus belli is no longer valid. Put down weapons and go home' - 'Oh...ok... Seems legit'.

I was so furious. And I'm still confused. How the hell this happens to me?