How come this county has both Muslim and Christian holdings

Clever, you found a way to create a merchant republic I hadn't considered before.

First, I think the only reason you need the soon-to-be Grand Mayor a county before granting a duchy is because the game won't allow you to grant a Baron-tier character a Duchy directly. Apparently you can get around that with claim wars. The city doesn't actually need to be in the county, nor the city or county in the duchy, it just makes for cleaner maps and less arguments between vassals over who should control what.

Second, unless crown laws (Medium or higher) prohibit it, the Grand Mayors are likely to try declaring trade wars on one another and possibly get Feudal Dukes (or even outside powers) to Embargo the other republic. They will also likely be plotting to steal each others trade posts and if the new republic gains enough posts they could threaten the old republics trade zone bonuses. That kind of internal strife costs money (and thus is inefficient), so it's probably better to quietly smother your fledgling republic.

Third, I don't think the game will let you revoke the city if he holds no other county or baron tier titles. You might be able to revoke the city if you grant him a feudal county then strip him of the city title. Of course, then you've created a Duke with a reason to hate you, and the two unjustified revocations will give you a -40 relation penalty with all your vassals. The simpler option, which will offend your vassals less (-20 instead of -40), would be to simply revoke the Duchy directly. The former leader of the republic will go back to being a regular Mayor and you will have a Duchy to hand out as you see fit.

The biggest thing working in your favour here would be adopting seniority succession, where the oldest dynasty member inherits your titles. With that, you stand a chance of having the King of England being the oldest dynasty member on your death. If that happens, then you take over the KofE as your player character, who also inherits the Kingdom of Ireland (etc), thus uniting the realms in a personal union. (From there, you can take a stab at Emperor of Britannia.) However, this is not guaranteed by any means; unless England adopts Seniority of their own accord, you have to be pretty lucky and/or murder-y to get the KofE to be the oldest dynasty member and then die before he does.

Aside from that, there's no mechanic I know of that's specific to merging realms shared within a dynasty. You're left with all the usual options that apply to non-dynasty realms: marriage/inheritance, vassalization, conquest. (Vassalization in either direction won't work for you since you're both kings.)

There's a couple of advantages you get from sharing a dynasty with your target ream ruler:

• You get a +5 opinion bonus among members of your dynasty, including the King of England. That's not a lot, but it will help you make political maneuvers.

• You'll also get extra dynasty prestige from having a dynasty member sitting on a large & important throne. That translates to some extra prestige for children born into the dynasty. High prestige means higher opinion boosts and better marriage options.

Both of these help, but neither are terribly significant, and both are usually dwarfed by the other factors that are specific to the characters in play at the time.

Conversely, there's two reasons why having a dynasty member as ruler of a target real makes the realm harder to absorb:

• If you're close kin (sharing a grandparent or closer), you run the risk of inbreeding; this makes absorption through marriage riskier.

• Dynasty members are always allies. Declaring war on an ally means that you must pay 50 prestige. If you don't have 50, you can't declare war (probably not a problem for a King).