Undervoltage protection strategy for Li-ion batteries
The only proper answer to this is really how much confidence do you need to have in your solution. If this is going to be a consumer product like you say, you will probably want to go after UL listing. I tend to always over-spec li-ion circuits, since failure with them is such a big failure.
For UL testing, the product will need to pass &quot;single-fault&quot; testing. This basically means that if any one component/solution fails, the battery shall not present a hazard. So, they would bypass the onboard battery PCM, and then overload/over-discharge/etc the system, and if you have no other protection, the battery would fail.
The &quot;over-kill&quot; approach that I have used is to have the onboard PCM, a PTC fuse, a battery manager (AP9101), and sampling the voltage of the battery with the MCU/gas-gauge like you suggest.
Note: the AP9101 is just an example, currently  using it in a few designs

The PCM is treated as the last-ditch safety, and the goal is to never need it.
The AP9101 is spec'd to activate before the PCM on under/overvoltage. Potentially also on current.
Since the AP9101 typically will have a wide range of current when it will activate, I rely on that only for really fast events, and near dead-shorts. For slight over-current events that are out of spec, the PTC fuse is relied on.
The MCU sampling the voltage range of the battery allows you to stop charging at a lower voltage than the AP9101 is spec'd at, allowing for longer life of the battery. (This really isn't always needed, but it's a free addition)

Using these three solutions allow for any one to be bypassed or fail, and will still protect the battery. Of course, this adds some extra cost which may not be acceptable, but if properly implemented, it will pass UL.

...and can it be then safely charged again?

Depends on the charger IC. Some won't allow a deeply discharged battery to be charged again, others have a slow ramp up that allows it.