Scaling defaults to having the pivot point in the geometric center of the selected vertices. With the mirror modifier on, this gives strange results because the only “real” vertices are on one side. If it’s just a matter of a tweak or two, I generally do a little bit of scaling along the mirror axis, then a small move along the mirror axis, which usually gets me close to where I want to go.
If you have more tweaks than that, it makes sense to change the pivot point to the center of the object, which you can do by switching to object mode, snapping the cursor to selection (Shift+S) then changing the pivot point to the 3d cursor (Keyboard period key or pivot menu.)
Then switch back to edit mode and start selecting and scaling. Be careful to not move the 3d cursor out of position.
Regarding the wavy line of connected verts you had highlighted, I think you have planes going out from those edges in three different directions. The Subdivision modifier is trying to smooth the planes toward all five connected vertices. If you just had four connected verts, you’d have a smooth curved surface, but that fifth vert is also pulling the surface toward it, and creates that little dimple. The only work around there is to not model like that.