Kindly read the details about Pro here:
You will have specific Header and Footer Builders to be able to create cool stuff, but it has a learning curve, You can see the environment of the builder here:
Check the tutorial above and see if such a functionality is for you or not. The X does not have the Header/Footer builder option and has the stock headers and footers of the stacks.
The important point is that you CAN NOT use the demos in Pro, the demo import functionality is not available and you need to build your website from scratch.
You do not nee Cornerstone anymore as soon as you convert to Pro as the Pro theme has exactly the same builder at hand called Pro Editor. You will have all the functionalities of the Cornerstone in the Pro Editor.
You will be able to create Header and Footer like the website you have mentioned, but you might need small CSS tweaks here and there.
The blog layout you have mentioned is not available in X or Pro, but you can use plugins such as Essential Grid which is available as a bundled plugin in both themes to achieve whatever grid-like the layout you like. For more information:
I suggest that you check this tutorial to get the hang of the details of the builders:
All the articles you may be interested in can be found in our knowledge base:
The Blog Post count is not related to the theme or stack you are using, it is purely WordPress related thing. Actually it all depends on how strong is your hosting service provider to handle stuff but generally, WordPress does not have any limitations regarding the count of the blog posts. You can start your research regarding this by checking This Google Search.
I want to mention that you can achieve what you want but it will not be easy if you do not know the options and stuff of the theme and builders. There is a learning curve here and you need to spend a little bit of time, but after you get the hang of things you will see the vast possibilities at hand.
If you are interested in converting from X to Pro kindly read this article:
Finally, we are here during the course of the development and will do our best to help you around. We will not be able to implement the customization of stuff that is not available in the theme out of the box, but we will do our best to give you correct suggestions, meanwhile, we will be available to guide you for the options of the theme for sure.