Best way to handle offline data in jQuery Mobile app (LocalStorage Vs Web SQL Vs SQLite Vs anything else)

I am building a jQuery mobile app which would then be bundled as native using phonegap. I am confused about which method to use while saving my offline data. It is an educational app which have data in following categories:

• Subject list
• Cards/small topics for each subject
• Question for each subject
• User bookmarks
• Some analytics data like time spent on each subject etc

Currently I have implemented some logic using localstorage storing data as a jsonstring

Main problem is:

1. How to edit a stored json string efficiently?
2. How to link data stored in different rows for ex. individual questions and their answers by different users
3. How much data can I store in localStorage before it becomes slow.