As I was looking for a solution regarding the second issue here (about the lyric extension line), I found this question. However, I’m afraid typing an underscore in the lyrics pop-over will get you a curved elision instead of a straight line. Since I found a solution for this issue on another question, I’ll share it here as well.
I believe the best way right now to get a lyric extension line for a single note at the beginning of a second ending is by using Shift+x while the note is selected, typing an underscore and then changing its position in Engrave Mode.
I do hope the issues with second endings will be solved someday, though, and we won’t need this kind of workarounds.