You have already answered your own questions.

1.  The code you have for question 1 looks good.  Is it not working for you?

3.  You can use the same code in question 1 to read line "n".  Just add an IF statement.  If vLines = n then vLineContent has the value of that line
While(FilePos(vFileHandle) < vFileSize) {
     FileReadLn(vFileHandle,vLineContent)
     vLines = vLines + 1
     If vLines = n Then {
              Do something with vLineContent
              }
        }

2.  And now you can read the last line by running both in series.  Run first code to get line count (vLines).  Set n=vLines.  Run second code looping until n = Linecount
------------------------------------------
n = vLines
vLines=0
FileSeek(vFileHandle,0)
While(vLines < n) {
     FileReadLn(vFileHandle,vLineContent)
     vLines = vLines + 1
     }
Do something with vLineContent
------------------------------------------

Note, samples above are not tested.