Only single-tile objects can be placed at a 45-degree angle and still be used.
Not correct. Two-tile objects like the big TV, stereo, loveseat/sofa can also be placed at an angle. As can three-tile couches. In fact, if you have a sofa and a TV at an angle directly in front of eachother, Sims will happily watch TV from it.
Just be aware Sims will walk through the objects, and the animations for sitting down are messed up.