After playing Samus Returns, Ridley stealing the Baby Metroid from Ceres as soon as Samus dropped it off makes sense. After running away and discarding the last of his cyborg parts, Ridley made a beeline after Samus' route and stole it right away.
Mother Brain killing the Baby Metroid seems pretty counterproductive, considering her plans revolved around the Metroids and she just destroyed the last one. However, it goes to show just how dedicated Mother Brain was to killing Samus. The Space Pirates could always find alternatives to Metroids, but getting rid of Samus forever was a prize too good to pass up.
We see in later games that the Galactic Federation was able to retrieve Metroid DNA from this incident too, so the Space Pirates would've presumably been able to do so with ease. Killing the last Metroid was merely a momentary setback.
Crocomire doesn't attack you if you don't attack him and/or approach too closely. Even if you hurt him once, he takes a few swipes at you, but doesn't charge. Crocomire could very well be a peaceful creature and just wants you to leave. Unfortunately, it's not possible to do so until you defeat him, which is just a further point of cruelty on Mother Brain's part.
Even more so, Mother Brain may have left Crocomire inside one of the only rooms in Upper Norfair where the lava from Lower Norfair is visible. And even more horror: Crocomire could have survived the Ring Out if the lava was from Upper Norfair and not the hotter kind from Lower Norfair.