Congrats on all that stuff you showed in the screenshots. And grats on learning how to use the vertex editing tool (not clipping tool) to make the corners of brushes meet up like that, but you needn't bother if the exposed edges are in the void. They get culled when the map is compiled.
However that technique is useful for internal geometry sometimes because it can neaten up portals and fix lightmap issues.
Don't concern yourself with portal efficiency too much in such a simple area. If you start watching the numbers like that you might do silly things to get it down, and harm visibility optimisation in the process. Visleaves are your friend. The more you have (within reason) then the more the map can be segmented up and limited in the player's rendered view.
Just a thought for you to go on with: If you want to use area portals to divide the spaces between your rooms, say you put one in the tunnel, there, which is good practice, then you'll probably need to put one in your sky light brushwork (the glass stuff in that big room) to seal that room from your 'pond room'. Simplest way to do that would be to make a horizontal area portal just below your sky light brushwork. For an example of a use of a horizontal area portal in a Valve map, check the 'courtyard' area in either team's base in 2fort.
And following on from what Tumbolisu said about texturing all sides of a skybox brush with the skybox texture: When you build your map, you needn't bother nodrawing any faces that are in the void, because they will be culled during compile. If you build a room out of dev textured brushes, or concrete or wood or something, then the brush will have your choice of texture on all its faces. But you can safely ignore the faces that are in the void, or covered entirely by other world brush faces, because of the culling done by VBSP that will get rid of them. Try it out in Hammer by making a test world composed of fully-textured world brush faces. You can even go further by putting in internal world brush walls, and flying around your compiled map in TF2 using noclip, to see which faces have disappeared.