Hi Harry,
Thanks for using our products. I personally started working with X as my second Wordpress theme around 3,5 years ago and I never looked back. Sincerely, my personal opinion there is no matching product on the market which will offer so much power flexibility and awesome support 
You might find sometimes stuck on the learning curve, but that is due the amount of power and felxibility of our tools, whenever that happens feel free to reach us so we can assist you.
That said,let’s go to your questions.
1- If you have installed X, altough that is not officially reccomended, you can Install one of the demos of X and than upgrade to PRO and you will have a demo as a starting point. You will install the demos in Wp Admin > X
Pro does not offer demos yet, We will be implementing a new functionality on the next days which will replace the demos offered by X and will bring even more power to the PRO Theme.
So basically, if you have X Installed you can simply install PRO, validate your theme just as you did with X and keep working and the only difference at this point will be thar you will have the benefits of the header and footer builder.
2 - At this point the only difference betwen X and PRO is that X offer Demos while Pro offers the Header and Footer builder,s, so you can watch X tutorials and they will serve you 100% on Pro
3 - Now my personal tips on how to start:
On X or PRO > Theme Options you will set the general styling of your website. so while using the classic header and footer you will set inclusively the style of your header and footer, button colors, text font color, links color etc…On Theme Options the settings will affect every page of your website…So for example if you have a site with a Blue and Orange Logo, you can set the links to be orange, buttons to be orange with blue hover etc…and this will make your website look good.
4- Your pages on X will be built with Cornerstone, On Pro Cornerstone was integrated with the theme and it is called Pro Builder, at this stage they look 100% the same, so Cornerstone tutorials are valid. I would suggest you starting using the classic elements , the classic elements are really good and have less settings and will follow the settings established on theme options, so as you are just starting, it will be less overhelming at this point not dealing with the flexibility of the NON-Clasic elements.
5- Unless you need a complex header, I would suggest you to not use the header builder until the end of your development, it is a brilliant tool, but you should concentrate on first learning Pro Builder and when you have a good result you can go for the header and footer builder, it is not hard at all, but it demanda a couple of hours learning, you will find youtube tutorials if you look there and also a lot of videos on our Knowledge Base :
Hope that gives you a good over view, good luck on your endeveour Harry and be sure you won’t regret, these are absuluytely brillant tools!
Let us know if you get stuck somerhwere or need help with guidance.
Cheers!