First of all, for this bit:

> ...the pre-existing margins interfere and act as unwanted text boxes.
> I can't find a way to remove them

Sounds like `Primary text frame` was ticked when you created the document. 

Normally, margins are just margins - innocent guidelines that don't get in the way. The `Primary text frame` tick box turns them into default text frames. Looks like it's ticked by default if you choose 'Digital publishing'.

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As for the other problem, there's two parts. First, getting a text frame across each spread. It's easy, and it sounds like you might already have figured it out, but here's the steps anyway:

- Create a 2-page spread master (page window > New Master)
- Stretch the text box across it
- When adding new pages, add them two at a time using this master
- (the more obscure bit) To get at and use the text frame from the master page on each page, **'override'** it (drag it from the master to the page) by selecting it **while holding `shift` and `cmd/ctrl`**. Otherwise it treats it as purely part of the background and won't let you select or edit it.

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The harder part is getting them to all link together into one story (assuming you want to do this). 

If you do need this, [read up on the various auto flow options][1] and see what works for your case. They're all a bit quirky, so the best approach depends on personal preference and the fine detail of your workflow. Might turn out to be easier to just link them manually.


  [1]: http://helpx.adobe.com/indesign/using/threading-text.html#flow_text_manually_or_automatically