Acting like you're entitled to some (vaguely defined) perfect solution isn't a substantive technical discussion. If you don't know what you're even asking for (conjuring nebulous 'clever people' to fill in your blanks), then you don't know if it's even possible or not!
There are lots of "good enough" range estimate solutions (long used on older cars), but there's no perfect solution that will never over-estimate the range ever ever ever (not ever). If you 'solve' this by making the range estimate wildly pessimistic, the driver will simply learn to mistrust-and-adjust, and then you're right back where you started.
Gasoline vechicals do this all the time with their range calculations. They typically exclude the last gallon from the range calculation so that the car won't stop immediately at 0 range.
I feel like if I crank the heat or start driving 80 MPH, it would be surprising if the estimated range didn't budge.
The straightforward worst estimate is to assume you'll immediately climb the highest grade road on Earth, against the strongest headwind on Earth, in the coldest conditions and worst precipitation ever recorded on Earth, until you run out of battery. That's probably not the calculation you actually want, however.
Ultimately the question is akin to tuning sensitivity vs. specificity in a medical test. There's a simple answer that can eliminate one class of error (set the tuning threshold to one extreme), but actually that's not the correct answer most of the time.
With the tuning set to the 99th percentile, the computer will underestimate range 99 times for every one instance where it overestimates range. That's not the absolute worst, but is sufficiently worst for you? Or do you want it even more pessimistic?
IMO the answer is cars should simply decline to provide any range estimate at all (just a battery percentage) until the user inputs a destination. None of this (fundamentally intractable) "guess if they're gonna drive up that nearby mountain or not" problem...