See the following link for one way to add a hover tooltip that is associated with your span. Its conceptually similar to your thought about adding a line. Either should work depending on the mode you choose for activating. I think regardless you’d want to set the alphas (transparency properties) for that added glyph to 0.0 so it is not visible from the user’s perspective.
https://discourse.bokeh.org/t/spans-and-hovertool/5171/6
Regarding the second part of the question, have you tried generating two separate hover tools and associating the first one with the trendline and the second with the vertical line/span. The HoverTool has a renderers property where you can list specific glyphs that you want to be associated with it rather than all glyphs on the plot.