For the 1st part - Horizontal reference lines can be added from Add-Ons > Reference Lines > Add reference line and defining an expression for the value to be shown. This assumes that your measure is on Y axis. Given that I am not sure why you would add a vertical reference line since that would make a reference mark on the dimension axis.
For the restriction on what shows up on the x-axis use the limitation feature for the dimension.
and this is using a limitation value of 2 on the measure - which stops the last two quarters from showing up on the X axis
You can select if you want to show the Others or leave it out.
Is this what you are looking for - if yes, hope this helps. If not, maybe could you share a picture of your chart to show the problem.