Blocking users from adding hosting adds complexity that could break your website. All of the security is on the registrar side, so it's something that would hurt more than help.
X10 now offers free SSL through let's encrypt. You just have to go to SSL settings (you can type this in the Direct Admin search bar) and turn it on.
When you sign up for a domain, you will be asked for the IP address of your server. This is the A record which binds your domain to a server. MX records can be added to specify where mail goes when it is sent to you. This is specific to the registrar you choose. Make sure your registrar lets you add this records, as x10hosting free does NOT allow you to add DNS.
x10 is indeed a registrar if you have an x10premium account. I love x10 and I've been here since 2008, but my preference is namecheap for domain registration. They have a ton to choose from and they also have extensive DNS management and other features that x10 does not currently support. There's no reason you need a domain if you're happy with a .x10host.com domain; you can just have a shorter URL and theoretically higher ranked matches on search engines if you do.
Javascript is a client-side programming language, so nothing is done at all on x10's (server) side. You will build your website on your personal computer, then upload the production version to x10. There's no option to build the website on the server because x10hosting free does not have SSH access, but that's really a pain, and it's much better to do this on your PC.
Since you have so many questions, I recommend that you take this one step at a time, and create new threads every time you have a new issue. To get you started, research why (and why not) you should have a TLD (top-level domain). Once you have a website, you probably don't want to change the domain at a later time, so once you pick something, be prepared to stick with it. If you want a TLD, then you need to find a good registrar; they should have privacy guard and DNS hosting at the very least.
