According to this writer, there is a "spectrum" of femininity/masculinity, that might be represented as something like a number line:
more feminine <-------------------|-------------------> more masculine
Freddy Mercury's features and personality allowed him to dress and act (present) himself anywhere on (along) this spectrum. "On" is generally more common for talking about positions on a spectrum, but "along" implies a dynamic rather than a static value.
You don't have to be familiar with the culture to get the basic sense of the difference between "glam drag" and "drag drag". "Glam" is short for "glamorous", and there are many images on Google showing "glam drag" performers. It seems to be a particular "look" based on a combination of makeup, costume, and hair style.
When English speakers repeat a particular word, with emphasis on the first word, it means an archetype or extreme of that word. For example:
He's a vegan's vegan -- he'll unfriend you just for posting a picture in a leather jacket.
She's a celebrity, but not like a celebrity celebrity. What I mean is, many people know her name, but no one asks for her autograph when she's out on the street.
In your example, a "drag drag" means Mercury could dress in whatever way is typical of drag performers. I don't know either exactly what this means, but it doesn't seem like an important detail. What is important is that he could appear attractive in many different styles.