Guard pages catch /some/ stack overflows / heap collisions, but not all. In particular, anything equivalent to an alloca() of more than a few pages, followed by an access to the lowest-address element of the allocated range, can skip the guard pages undetectably.

The additional tool need is stack probes, which modify every allocation of a stack frame that may go past the guard pages (large function frames, alloca, etc) to also include a probe loop that reads from each page, guaranteeing a page fault on a guard page. The catch here is that stack allocations go from O(1) to O(size) time.

Sometimes. Guard pages are not always used, depending on your OS or C compiler for example, and on how large your address space might be. Especially 32bit applications are quite limited in that regard. Embedded always lags behind a few decades. Commercial applications are often used long beyond their due-by-date, and vendors never seem to care about that stuff anyways.

If there is only one or a few guard pages, you can sometimes still get your exploit to work by "jumping over" the guard page with a sufficiently large stack allocation that isn't used. But that is admittedly rare.