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'.

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.

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 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.

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'.

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.

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 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.