Re: It isn't that hard
I worked directly in the MTD team defining/implementing the API - against the HMRC Policy Team's requirements. They really are incapable of thinking about data, only about the paper form with 9 boxes. We did our best to protect you/us all from them and the 1000 year old organisation around them.
Re: spreadsheets - absolutely acceptable, but there must be an automated i.e. code process that sends data to the API. No copy paste into handcrafted JSON and scripted REST requests, sorry.
What must be sent via API every 3 months is pretty much what you enter on the website now, but only for that 3 months, and to be sent within 1 month of the end of that period else PENALTIES.
You need to maintain but do not have to provide the raw transactions i.e. the spready unless they ask, for example in an investigation.
In the case of VAT, completely trucking pointless with such a low error/compliance rate in the existing web submission system. But it will prep people nicely for the real prize - MTD for self assessment income tax . Yep you're going to have to do your income tax return every 3 months too.