So for the shock of the explosion to force the saucer into the planet's atmosphere, Troi would have had to deliberately position them between the drive section and the planet. She simply could have flown... gosh... away from the planet.
I don't see this as a biggie. We witness the saucer getting only a very short distance away before the core blows - it wouldn't make any difference if some other course were selected because the saucer would in any case still be sitting right next to the stardrive section, not having been able to make the course choice "count" yet.
OTOH, plunging into the atmosphere might be a valid tactic for protecting oneself from a nearby warp core explosion. That's the only "shadow" available anywhere near the kaboom, after all.
As for turning away when you are being pummeled... What could possibly be wrong with that? Turning doesn't detract from Riker's ability to fire back (except with bow torps, but we know that those can't be used at close quarters anyway - only Klingon Birds of Prey ever fire torpedoes point blank, and those green things never make a big bang like the Starfleet photorps), and it gets the wounded and still vulnerable throat of the ship away from the Klingons' line of fire.
Having Troi pilot the ship through the battle doesn't lend itself to excuses, though. Sorry.
I didn't criticize Troi for turning around. Riker told her to get out of orbit, which pretty much does involve turning and one assumes the Bird of Prey is both faster and more maneuverable than the Enterprise anyways. If they wanted to get behind them, I suspect they could.
But the saucer separation scene, you clearly see the the saucer turning when it's flying away from the drive section.
Boy, if she had gone left instead they'd have been just fine. The shockwave of the drive blowing up didn't do anything more than just push them. And they did put some distance between them... as so.
And as we don't see the planet anywhere in this shot. She had to have been pretty much flying right towards it. So had she turned left when the separated, it would've pushed them away.