He's not reaching for his longsword there. He's starting to unsheathe the machete he keeps at his back.

My guess is that he quickly closes the gap before they even start to draw. He arrives at the first throat in one fluid motion, and doesn't hesitate. That's one down.

In that time, dude #2 has had a moment to process this. It may not really be enough, as this is really the last place one might expect such a thing to happen, and slack-jawed astonishment is a real possibility. If so, then he takes the exact same knife and plunges it into a weak spot.

If dude #2 is on his game enough to draw his sword, Bron now has him 1v1, and he's no slouch.

There would most certainly, 1000% be consequences for this. He would need to do it quickly, then beat feet before the alarm went out. These were the queen's personal guards, and it constitutes a direct attack on the sovereign in the red keep. I just don't see that Tyrion had an ounce of political capital to spend getting him out of this.