The problem:
I need a device agnostic (e.g. HTML5) solution for storing and querying 250,000+ rows of data offline on a phone or tablet type device (e.g
I would tell you to use Corona for it . It's a private Platform used for crossed-mobile applications which has support to SQLite .
Pros
Cons
I paste here what they say about it:
Corona includes support for SQLite databases on all platforms. This is based on the built-in sqlite support on the iPhone, and a compiled version of SQLite on Android. Note that this increases the size of the Android binary by 300K.
SQLite is available in all versions of Android, iPhone, and iPad, as well as in the Corona Simulator...