From my few experiences of travelling on the NLL, I have yet to encouter a
ticket check despite all trains having a guard present to open/close doors
at each stop. A lot of the stations on the NLL are unstaffed and some don't
even have facilities to buy tickets (e.g. on the Gospel Oak to Barking
line). Isn't there a penalty fare system in place on the NLL, or does this
not apply to stations where one cannot purchase a ticket beforehand, such as
above?

If there is no ticket office, you are supposed to use a machine (where
provided), if there is no machine, or the machine doesn't have the
ticket you want, you are supposed to put some money in the Permit to
Travel Machine (I only ever put the minimum (5p coin) in these machines
because I've found them to be quite dodgy in the past). If there's no
Permit to Travel Machine, you're supposed to get on, and pay at your
destination or interchange station (if time allows).

The reason you may never see a guard on the train could be because he
couldn't reasonably get round before the next station. I don't know if
the driver opens the doors and the guard closes them (like Southern), or
if there are door open/close panels by every doorway, but it's very
difficult to get far enough along the train *and* get back between
stations on the NLL.