He is an American Citizen, holds a US Passport and here on a Spousal Visa.
Then question 1 is no, and, of course, question 2 is yes.
Again, since he is a US citizen, the US will always be a "country of residence for tax purposes" thanks to the unique US CBT (citizenship based taxation). Forms such as these are made by the institutions themselves and are meant to cover all situations (OECD CRS, EU Savings Directive, FATCA from the US, etc.). It's all compliance based (and a CYA for the institution) and are, at times, slightly confusing due to all the varying situations.
I can't comment on your 2016 query (I assume you mean for the US) as we have insufficient information, but if he had NO income for 2016, from anywhere in the world, then the CPA
might be correct. If he had no US income, but income in the UK during 2016, then he may well have to file a US return for 2016.