Hi Tau Myx,
Welcome to Affinity Forums
No, all "lines" (segments) that conform a shape must have the same attributes (width, color etc). If you need to change the attributes of one segment you have to break the shape at the nodes that surround the segment you want using the Node Tool, selecting the nodes and clicking the Break Curve button in the Action section in the context toolbar (it's the first icon). You should now be able to select just that segment with the Move Tool and change its attributes.
No, you can't bend the unclosed line of an open shape. It's always a straight "line" connecting the start and end points.