Fastest
It is often the case that wee machines can take more abuse than bigger machines. This is because they dont have much weight or power behind the tools so damage is less. My lathe is big and heavy and fairly fast, the saddle, crosslide and turret/toolpost probably weigh in excess of 400KG (880lbs approx) so if they smash into something at 10m/min the something has to be fairly solid to withstand it. Then the problem would be all that force has to go somewhere so my turret would take the brunt of it. Fortunately I have only had minor crashes during the learning curve of CNC lathes and just smashed up a few tools, the worst was the threading tool that is an upside down type and costs about £120
thankfully I only did that once
Tool offsets and work offsets are all dependant on how you have things set up and how repeatable your replacing of tools can be. With a toolchanger you can set upp the tools to have the correct offsets in relation to your master tool, then all you need to do is find where that tool is in relation to your stock. Zero position on the X is always the centre line of the bed and Z is usually the end of the stock or at least the part you are making if the stock will need facing. With accurate homing you can have all your tools set up so that if you command a 20mm dia move it will go there no matter which tool you have in the slot but obviously that needs a means of placing the tools in the slot accurately and be repeatable.
Now bacjk to your first pass, is the spindle stable when the first pass is happening? if not then it is likely the axis is reacting to the varying spindle.
Hood