VAT MOSS and the EC Sales list are two different things and both must be reported. If you have a letter from HMRC stating otherwise I think you may have asked the wrong question.
VAT-MOSS is about reporting to EU tax authorities (through a single authority) sales to consumers.
The EC Sales is is about sales to businesses. That is, sales to entities able to enter a VAT number. It is essential that these are reported separately because they are subject to the reverse charge mechanism (a separate and preexisting provision of the tax legislation). Without it tax authorities cannot determine when they should be expecting VAT revenues on purchases from businesses in other EC countries.
You are correct about SAF-MOSS but until the first audit queries begin we'll not know what will and will not be accepted.