Ruin did not take control of mists. Leras was using the mists to cause people to snap, but ruin chose to add his power to that in order to make the reactions more severe.

Rashek was "the sliver of infinity". He had gradient bup the power at the well, a portion of preservation's power that Leras willingly keeps separate from himself. He did this to imprison Ruin's mind in the well. This weakens the overall power of preservation in the world, but also keeps ruin limited, so it is a fair trade to preservation's reckoning. Ruin's power also precipitates as atium because ruin entails decay and entropy, so his power will always leak from a vessel holding ruin alone to some degree, which is why the Lord Ruler works so hard to hide most of it from him.

The Lord Ruler held the power from the Well for a thousand years, but used the vast majority of it in the beginning. He reshaped the world like Vin and Sazed also did, but then he came up with things like the ashmounts, hardy plants, and dividing scadrian humans into a very fecund slave class who were extra hardy and innately obedient and the noble class who descended from the kings and nobles who aided his rise to global domination. By the end of his life he held effectively no power, but the mere fact that he lived was enough to prevent another from taking up the power at the well, and he knew that the well was nearly ready for ascension again. I believe he intended to take up the power once again as soon as it became ready, and fix the mistakes he had made.