The first picture looks like it's caused by a lack of cooling. On smaller layers, the head lingers for too long in one spot and the layer doesn't have enough time to cool properly before the next layer is laid down, resulting in a melted look. Printing two at once will give the smaller layers enough time to cool.
Regarding the bottom step, it may be (at least partially) caused by the first layer being smooshed slightly into the bed and therefore not as tall as subsequent layers. This is necessary for good adhesion to the bed.
Regarding missing steps on one of the Z axis motors, make sure your vref for the stepper driver is set correctly. You could try upping it slightly and testing the Z axis travel to make sure it's smooth and doesn't miss steps.
[
3DKarma.com] - suppliers of quality, affordable 3D printer kits and filament for the UK market.