All of those methods work, as does the Move tool. With the Move tool active and nothing selected in the model, hover the mouse over an unconstrained end-point of the edge. Click-and-release the mouse, and start moving the end-point in the desired direction. You can click-and-release a second time to determine the new location of the end-point, or you can type a dimension and press Enter (in which case the end-point’s new location will be in the direction of movement, with the specified distance from its initial location).
Adding an additional edge to extend an existing edge works OK visually, but personally I wouldn’t like the resulting geometry that contains two co-linear edges. I haven’t tried this, but perhaps one way to cause SketchUp to merge the two short edges into one long edge may be to draw a third edge at the intersection of the two short edges, which heads off to the side (arbitrary direction, arbitrary length). Then delete that off-to-the-side edge. The deletion may trigger SketchUp to merge the two remaining co-linear edges.
I think this highlights the uncertainty. Sometimes joining two colinear lines with Weld fails for no apparent reason too. I tend to use the Move tool, especially when on axis. It wouldn’t occur to me to use the Scale tool but I guess that would effectively do the same thing.
I’ve worked with other CAD/modeling programs including many years with Vector Works, but am relatively new to Sketchup. I find it surprising that Sketchup doesn’t allow using the Move tool to simply ‘move’ the end point to make a line longer.
Even Sketchup’s own instructions claim you can hover over the end point and move it to extend a line, but this moves the ENTIRE line, or if it’s part of a larger polyline, it distorts the polyline. Is there something I’m missing?
Is it possible that they made it impossible to do one of the most basic things in drafting or modelling - extend one line to meet another?