Between a healthy dose of stewardship, gavelkind and a little bit of over extension it was quite easy to hit 6 counties, considering I even saw an AI controlled Firenze with 5 counties I assume it is possible to get a 5 demesne limit.
On my second run through I see now what you mean with the religions, I think in my first run the religions were messed up a bit due to a few counties having been inherited by someone on the opposite sides of the world; in my second run I didn't have the 8-10 religion chaos in Italy, pretty sure it was only 3 with a fourth on Sardinia.
The culture spread is very cool btw, playing as Navarra just now I was the first to get the Iberian culture, so roughly a third of modern day France ended up Iberian, whilst West African managed to get nearly up to Iberia (along with a random Hispanic culture country when Pisa somehow inherited La Coruna).
I think I misunderstood you the first time; I thought you meant that as a count you can't create a duchy when you have more than 5 counties. However now I think you meant the demesne size limit and not the actual duchy creation.
I haven't touched demesne sizes for dukes, so they should be the ck2+/vanilla values.
Do you use the 0.24 version btw? As the others mentioned problems I couldn't reproduce, I was wondering whether it was working with you.
On the religions, that may also have to do with the version. Now the random lists are gone, which makes sure all basic religions sprout an equal amount of time. Also neighbours now also convert along. It was actually a bug (I wanted it for poly's and mono's), however I liked how clean it looked, so I kept it in.
If you have any further comments about the religions, feel free to continue mentioning them.
On the cultures, thanks for the compliment. I am very proud of the bubbles and what Girafee did with the mingling