As a driver who has tapped drop brakes to slow down due to interference on the course (aka squirrels humping in my line during freerolls), I can assure you that it is possible to use those brakes to trim your speed. I'm not sure how it's possible that they wouldn't, unless you had a brake design that was either on or off with nothing in between and no way to disengage until the buggy is at a complete stop (is that even plausible?) Hitting the brakes at all during a race is going to throw your time anyway, so if you had to slow down to avoid collision it wouldn't be much worse if you ended up stopping altogether.
With that said, it is extremely difficult to judge whether or not you can make a pass or whether or not you should just suck it up and ride your brakes (which, if you are the second buggy in a three-buggy heat, isn't optimal, either; who knows how close the buggy behind you is, and if the driver is going to be able to react to a sudden decrease in your speed). The pass tests that get conducted usually do not happen with both buggies at full raceday speeds, and very rarely get conducted with the buggy/driver/heat combinations that will happen on raceday. I might know how to pass Conquest when driving Mirage if Conquest gets a walking bump over hill 2 and I get a full-out shove and I know the Conquest driver's line because we've been coursewalking together for two years, but that doesn't do much to simulate the speeds and trajectories of what happens on raceday.
The other thing about the pass tests that bothered me is that the valid passing zone defined for the pass test does not cover the entire freeroll. If you are only allowed to pass between the first transition and the monument for the test, then presumably that means you should not be allowed to attempt an earlier or later pass on raceday. I can certainly understand not encouraging people to bash their buggies together in the chute during rolls, but if you're not even going to acknowledge chute passes for the pass test, then you shouldn't allow them on raceday, either. Passes can happen anywhere. If there was an explicitly defined 'passing zone' for raceday, meaning if you pass outside of that zone on the freeroll you are DQ'd for reckless driving, then it would be much easier for drivers to make that judgement call.