That's like expecting a doctor with rank of commander in the Navy to be able to pilot a submarine. They're different areas of training and it wouldn't make sense for that person to be called on to drive. It was just some nut at Paramount decided it was a good idea to put Sirtis at the helm for dramatic effect.
We've seen Riker and Picard both jump over the bannister to fire weapons while simultaneously staying in command and giving orders, so it's not as if it's unprecedented. Heck Picard captained the Enterprise from the helm specifically one episode.
Well big surprise, Troi crashes the ship. And before you say it's not her fault consider this. After the saucer separation, Riker told Troi to get them away. Also remember space is three dimensional. 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.