No, the disk space is shared between the domains you have hosted.
Yes, the bandwidth is also shared between the domains. Practically you get in total 10 GB monthly bandwidth for all hosted domains.
Yes, you can create e-mail accounts for both domain names and can also decide the number of the e-mail's you want to create for one domain and how many for the other one.
The same thing is valid and for the sub-domains. You get the possibility to create 100 sub-domains which is shared between the two hosted domains (in case you host two domains) so you can chose how many sub-domains to create for each domain name.
Yes, you can order additional MySQL database(s). To view cost of additional MySQL database please check hosting add-ons page.
Yes, you can order additional add-ons for your hosting needs for everything disk space, mysql database, bandwidth. Read more about hosting add-ons and see the list of the add-ons you can order for your hosting.
It is not necessary nor we restrict to our customers to register the domains with us in order to use the Master Plan. You can have the domain registered wherever you are willing, to host the domain with us it is only required to point two name servers (NS) of your domain name to our name servers (ns1.hs-hosting.net and ns2.hs-hosting.net).