MightySites can allow all sites to share certain database tables, all users tables should be shared in your case: all slave sites will load users tables from database of master site, hence all users/usergroups/user-usergroup relations will be always same across all sites, ...

... files/database structure. Joomla doesn't support multi-tenancy, hence running multiple sites requires knowledge of which database data can be shared and how it's used.

Domain B will be slave in this case because it will load K2 and Joomla users from database of master site Domain A.
Both sites will use same tables, so all K2 database data and users will be same on both sites.

Note that K2 images won't be shared if each site uses own Joomla installation.

Yes, it's possible: Joomla users and single jomsocial users table can be shared, other JomSocial tables can be separate per site as usual.

Sam2Joom doesn't use native Joomla code, so it's database contents neither can be shared between sites with different databases nor it's database contents can be separated for sites with same database tables.

... data from master site). All sites should use same Joomla version and same versions of these components installed. Additionally all users should we shared as well. Note that images like avatars and photos won't be shared because each site uses own folder. You will need to setup symlinks to images ...

... the issues. Recently, I have two websites: www.website1.com www.website2.com Bouth are independent websites but sharing some tables. One of the shared tables is the Users-table. Another one is the easysocial-table. Now I created the single-login on website2. I turned the single-login on, filled ...

... User A registered on Site A → User's record is available on Site A or B for login What happens of user B registers on site B or C...? If users are shared - user record is available on all sites which share users. What happens if the user account is deleted on site A or B or indeed C...? If users ...

... - in most cases MightySites won't help you. Please note that MightySites doesn't sync or replicate database data. Users and subscriptions can be shared: multiple sites will use same database tables for users and subscriptions (depends on subscriptions component - it should use native Joomla database ...

It depends on how would you like to keep them.
If articles are shared - better share categories, otherwise categories IDs can be messed.