It would depend on how their Mental Attributes are. Bear in mind that Matrix Attributes are fully fungible in 6we. The commlink with D/F 3/1 also has a D/F of 1/3 available at a moment's notice.
Sure, but isn't hacking resistance Data Processing + Firewall anyway? Making that the same either way.
It depends on the action in question. Sometimes Intuition or Logic is called out in place of a matrix attribute. Backdoor Entry and Brute Force both call out Willpower as an attribute for the defender... and if you're trying to hack a car odds are you need to do one of those two things in order to get User access to the device.
And also, when directly protected you can use your mental attributes to defend against matrix actions.
Which I've never quite understood in this sort of scenario, by the by. What is actually going on in-world here? Is the commlink beeping and saying "argh I'm being attacked" and the person is actively doing stuff to try and defend it? Does that mean the user is always aware that a hack attempt has happened?
It is what it is. I like to imagine it as your "matrix discipline". Do you reflexively dismiss notifications from your commlink warning you that your software has updates available and perpetually leave your commlink open to exploits? That's a game manifestation of low Willpower, imo.
I've seen cases where writers in missions (5e, for context) said that an unattended car or a stolen commlink still gets to roll its owner's Willpower because the owner was the one who configured its matrix permissions.
Either way, I'm a fan of the concept of the GM rolling the defense test if it's a PC getting hacked. I'm an old school GM though... the player not needing to know anything the character doesn't notice is the right and natural state of things by my sensibilities

You wouldn't ask a player to roll Perception to see if they notice a Secret Door, would you?

You could of course instead say that rolling a defense test does cause AROs to start blaring warnings to the owner, and the Willpower represents how focused they are in engaging in a game of popup whackamole with the hacker... but this is directly contradictory to the sneaky sorts of hacking actions that aren't supposed to alert the owner to what's going on.