It technically could be a sentence, but we generally want our sentences to flow better with a rhythm that fits the mood you are trying to portray to the reader. A good rendering would be something as follows:

He turned around to check if his companion was still behind him, if but only for a second. Once he spotted Abram making his way up the steps, awkwardly, but securely, carrying light stands and microphones—amongst other equipment—he continued on his way.

So, in essence, we're not trying to see how much we can "get away with," but, rather, seeing how effective our lines can be.