Support just replied with the following - so between the two, hopefully I can get this project back on track. Thank you again and I will let you know the outcome.

The internal error is being caused by the last three pages on your document, 6 - 8, and specifically the "Page Background(2)" layer. This is also causing the extra page layer to be produced when you extend the page. Do you know how this happened in originally?

I'll forward your file to the developers for them to have a look at and try to establish why this is happening. In the meantime, if you delete the layers from the last three pages, which will unfortunately delete the page and pasteboard background layer from your other pages, which you will need to right-click > Page Background > Reset and reconstruct it.

The navigation problem is being caused by your header group covering the buttons up. If you move it to the MouseOff layer, and then select the buttons and bring them to the front (Ctrl + F), they should start working.

The ecwid widget seems to extend for me, however the contents still appear as one row of information. If you right-click the widget > Web Properties > Placeholder and change the code where it says:

views=grid(1,4)
to
views=grid(2,4)

It then expands onto two rows.