Question: There are many versions of Vedanta available and most of them have drastic differences and contradictions. 

**So is Vedanta is a loosely coined term or does it have a concrete definition?** 

**What is its relation to the philosophy?** 

If veda-anta is something **beyond/other** than the Vedas then it cannot be called as **vedanta** at all. 

**Further, there are no divisions such ad Veda-aadi, veda-madhya then how come we have something like veda-anta?**

If its a part of the Vedas then these philosophies also form the part of the so called KarmakAnda.

Answer: 
**Markandeya Purana;**

[Here](http://www.pushpak.de/markandeya/markandeya_en.pdf) is Markandeya Purana unabridged in English.

[And from here](https://ia801000.us.archive.org/2/items/markandeyapurana/markandeya_purana_hindi.pdf) you can get it in Sanskrit-Hindi (Sanskrit original slokas with their Hindi translations).

**Devi Bhagavatam**

From [this page](http://www.astrojyoti.com/devibhagavatamindex.htm) you can read it in unabridged English translated by Swami Vijñanananda. 

**Some other Puranas that have information on Goddess Durga.**

As is well known Durga Puja is quite famous in Bengal. And here they follow mainly four Puranas for the worship procedures apart from following the Agama scriptures. And, they are:

1) Devi Bhagavata 2) Markandeya Purana 3) Kalika Purana and 4) Brinnandikeswara Purana.

Of which , the last two are considered as upapurans. So, apart from the Markandeya Purana and the Devi Bhagavatam, the two upapuranas i mentioned also contain a lot of information on Devi Durga but i think it is hard to find them online.
Gita Press Gorakhpur is an online site that offers books on Indian Religion and Mythology at unbelievable prices. You should check that out.