Raymond says he spoke to his mother at one, the hottest part of the day. A common mistake is to assume noon to one is the hottest part of the day, when it is actually two to three pm, regardless of where in the world you are. The heat continues to build until that time then the weaker sunlight in the later hours causes the temperature to drop.
Pandora's "Box" was actually a Jar. The idea of it being a box comes from a mistranslated of the ancient Greek word for jar, pithos.
If Lady Boynton had been stabbed to death when and how Poirot's solution proposes she was, there certainly should have been fresh blood on her person when her body was examined on the scene after being discovered - not just the dried blood, from hours before, engineered to make it look like she was killed earlier.