1. How to change Blog page layout style to list or grid or masonry?

Find option here : Appearance > Customize > Blog > Entries & Archives > Posts Layout Style
Refer for more details : http://docs.tanshcreative.com/charvi-blog-settings/

First post will have standard layout by default.
Standard First Post can be ‘ON / OFF’ to have all posts in same layout.
Find option here : Appearance > Customize > Blog > Entries & Archives > Standard First Post

2. Where to add links for ‘Social’ above footer?

Find option here : Appearance > Customize > General Settings > Social Share Section Above Footer

3. Where to add links for ‘Social Icons’ in header?

These are via widget. Social icons can be added / removed as per choice.
Find it here : Appearance > Widgets
Drag ‘Custom: Social Network’ widget to preferred sidebar.

4. How to re-use some element / section from one page in another via Page Builder?

Take advantage of page builder’s ‘Template Library’ feature.
Please check – How to Save Sections as Templates? – from following link.
How to Save Sections as Templates?