I mean legally they can of course. Technically they can probably cut it off for 95% of people relatively easy. People who are technically proficient will almost always find ways around pretty much any block.
It depends. If the phone's locked then that's a whole different kettle of encrypted fish, as shown by the FBI case last year where tried to get Apple to unlock a phone.
For me, this is the real core of it. While not having an overbearing government is obviously very important, any policy that allows authorized forces to break encryption will, eventually, allow those without that authorization a way in.
They might well be able to, but it's probably not very easy. The maths behind these protocols is well understood and very hard to break, and while there may be a flaw in the protocol itself, WhatsApp is using an Open Source protocol, meaning any security researcher can look at it and find the...
As far as I know WhatsApp is using end-to-end encryption, meaning they don't get a version they can decrypt. That's the point of end-to-end encryption, and why it causes security services such a headache.