Please find an attached file with some possible solutions to the problems you illustrated in your screen grabs as follows:
UV Mapping - There is occasionally an issue with automatic box UV mapping bridging segments in different inputs. To work around this I've restructured your style to avoid the problem by using geometry only in the default inputs for horizontal elements.
Sliced Geometry - Setting the default mode to adaptive will automatically scale segments slightly to ensure no slicing takes place
Incorrect geometry on corners - unfortunately this is a limitation of this kind of joinery, it wouldn't be possible to create this joint with a simple mitre in real life. This kind of joint would only really work with handrail with a circular cross-section or other profiles where there is no need for a particular face to stay pointing upwards. One way of solving this would be to come out horizontally immediately after the 90 degree bend and then angle downwards. Alternatively create a wreathed handrail
style that that will create a continuous curve to follow the complex angle. Which approach you take depends on the style of handrail you're trying to create. The file attached uses the latter technique.
Hope that helps,