Thanks for asking - there's a few ways for custom tick labels - the best place to look would be under SChartAnnotation - you can add annotations to the chart at data positions, and set clipToBounds to NO, which should let them move onto the axes.
For drawing circles at arbitrary points, you could again use annotations, overriding their drawRect method in a subclass and drawing a circle using CoreGraphics. Our crosshair draws using a similar technique.
For your third question, we currently only support tracking a single series with the crosshair, so if you wanted to draw circle targets on multiple curves, you would have to do this manually in a crosshair subclass, I'm afraid. This is a feature we have on our roadmap, however I can't give you any solid dates for this yet.
And finally, yes - you can override the -crosshairShouldKeepTracking method on SChartCrosshair and return false. This will prevent the crosshair from tracking once you start panning.
I hope this helps! Let me know if you have any other questions.