Triangle reveal

The easiest way to reveal that triangle is by simply adding a linear wipe to the shape layer. You can even reveal the entire graphic. All you have to do is add the Linear Wipe effect from Transitions, animate between 100% (entire layer gone) and 0% (entire layer visible) then adjust the angle of the wipe to match the top edge of the triangle.  The start and endpoints will have to be modified a bit because Linear Wipe works on the layer width. In this example, I reveal the entire red shape. It took about half a minute to create the animation.

Cropped screenshots that don't show us the timeline and the modified properties of the layer do not give us much to go on. I'm also not exactly sure what you are talking about when you describe offset paths. 

Changing blend modes will let you see through the layer. Adding a mask or a track matte will poke a hole in a layer. If the camera is moving you are going to need to do some motion tracking or image stabilizing. If the shape of the triangle needs to be animated you will need to animate the path. 

I can give you better advice if you post a screenshot that shows the triangle layer with all the modified properties revealed. Just select the layer, press "uu" and post a screenshot of the entire UI. Then describe what is happening in the shot and what you want to see.