Can I switch in the same scene between 2d and 3d
display modes and and different projections(orthogonal and perspective) ?
For example - to print out text and to draw the background in orthogonal 2d and to draw the scene objects in 3d perspective mode ?
Yes you can can change the projection at any time you want, for example set a 3D projection and render the 3D stuff, then change to an orthographic projection and render any text or other 3D stuff.
Just remember that when you change the projection you (may) also change the depth buffer settings, so make sure that you either disable depth testing or set a depth value for your 2D stuff so that it doesn’t end up under the 3D stuff.