If you want to count examples you're reasoning about this the wrong way. The question is what way leads to a higher probability of making a mistake, not the existence of discrete examples. Considering that one way, Valgrind works out of the box, but with your way, it doesn't, I think the answer to that question is quite clear. Even if there weren't tools like that, it is easier to read code and understand pointers' lifetimes when they're being handled individually, instead of having their lifetimes be part of some far-off region.
The whole point of using regions is not to be worrying about pointer lifetimes. And you don't need Valgrind when you have regions since you aren't going to leak memory by forgetting to free something. Valgrind solves a problem that doesn't exist when you use regions.
If you're really worried that you'll leak an entire region worth of data just allocate the region with malloc instead of mmap and then use Valgrind to tell you what regions you aren't destroying.
