I'm not sure about the answers to some parts of your question but here's the easiest way to define the default program for a particular type of file:
Right-click on a relevant file. Choose 'Open with'. Click to place a tick in the box labelled 'Always use this program to open these files. Scroll down to the program you want to associate with that file type. Click on it and then click 'OK'. (If the program you want isn't in the scroll-down list, click 'Other', navigate to the location of the relevant program and double-click on it).
Chris