1. Desktop version is used for tablets by default. It's possible to assign another template in MJPro, but anyway default setting is to use desktop template.
2. Just assign that modules to specific pages (in a usual Joomla's way). If you like to disable some modules for tablets with desktop template, use MJ's Module manager (it is included in MJ Community version as well).
3. Yes, add styles to template's css/custom.css file (this file is not overwritten by MJ updater).
4. Yes.