NXT's account control is based on the phasing feature. It is very different than Bitcoin multisig.
When submitting an NXT phased transaction the user, in addition to the transaction information, attaches a voting model and a block height stating that in order for this transaction to be executed another approval transaction(s) matching the voting model needs ...
According to NXT wiki:
In Ardor, only child chain transactions can be phased. Therefore, when account control is set for an account, it can no longer submit Ardor chain transactions.
In other words: account locked for child chain is locked for Ardor chain to.
See for details: Account_Control
Yes, if you don't want the hassle of registering or giving your details to a website an ATM would be the better choice. Note that ATMs usually charge more than an online exchange.
They give you a bitcoin address and its QR code as well as a private key and its QR code.
The private key is all you need to have control and access of your coins on that ...