From a human factors perspective, whether you can know what it will do isn’t sufficient. What matters is whether you will know when it matters, even if you’re tired or distracted and panicking because you’re rapidly approaching a kid that stepped onto the road.
I would expect the likes of Volvo to make the traditional brakes pull in when regenerative braking lacks power. They tend to have a huge human factors department.
Regenerative Braking comes in two forms, passive and active. The notifications regarding limited Regenerative Braking Capacity are specific to passive braking (e.g. backing off the accelerator, lifting your foot off all pedals). Active braking will working using either Regenerative Braking or traditional brakes so what it's doing is irrelevant as the behavior is identical to the user.
So in your scenario where an individual might need to actively brake, they would be able to without issue. Where it might be an issue is when you're driving in bumper to bumper traffic and you're expecting regenerative braking and it's not there and you roll into the car in front of you. Then again, you don't make it very far down the road without noticing a difference in the accelerator pedal characteristics in limited regen mode.
The way I understand this is that, most of the time, users will be braking by removing pressure from pedal A, and only in exceptional cases they would have to do it by applying pressure to pedal B.
If so, I think you cannot expect humans (most of them even amateur drivers) to make that switch under stress, no matter how well you signal that the car requires you to use mode B.
The Gen2 Nissan Leaf has an "e-pedal" that works differently so I can't speak for that, but for the Gen1 Leaf and a 2018 Model 3 you absolutely use the brake pedal. The difference is that Regenerative Braking just changes your timing when switching from Accelerator to Brake, it doesn't eliminate it entirely.
Have you driven a standard transmission ICE? Downshifting into a lower gear is a very similar experience to regenerative braking. It's an aggressive deceleration that will slow the car from high speeds but it's no replacement for actual brakes.
For someone inexperienced with an EV, the lack of Regenerative Braking would feel like driving a normal car. For someone familiar with it, you're immediately aware of it's absence. In an ICE you coast by taking your foot of the accelerator, in an EV you coast by applying 10% pressure to the pedal (I made that % up). The reason you need 10% pressure to coast is that anything under that amount is where regenerative braking functions. So there's a weight to the pedal that isn't there when regenerative braking is gone.
It's hard to explain without driving an EV but it would be like using a computer mouse without the scroll wheel. You're keenly aware of it and if it's not there.
I would expect the likes of Volvo to make the traditional brakes pull in when regenerative braking lacks power. They tend to have a huge human factors department.