In all of the Trek shows, the "order of succession" is established only for the top three, sometimes only for the top two. And probably rightly so, because in all of the shows, there are plenty of background extras who carry higher rank than the main characters from #3 down. Those are the likely people to succeed the #3 hero.
Of course, we basically never see anybody succeed the #3, because that would mean that the ship has taken horrible casualties or that a great many heroes have deserted their posts. Instead, we often see a situation where #1 and #2 beam down and do NOT leave #3 in command - but instead "give the conn" to some junior officer who happens to be the highest-ranking and/or best qualified junior officer present on the bridge at that time. This makes good real-world sense: if top officers feel confident that they can leave the ship, they should also feel confident to leave the bridge in the hands of a qualified but not particularly senior officer, rather than bother the more senior officers who would theoretically be next in the line of "succession" but who no doubt have much better things to do.
In the real world, sitting on the center seat of a warship is often boring and tedious work handled by junior officers, except when there's something important going on. And today, the skipper is more likely to leave the bridge than to enter it if battle is imminent, because modern ships have separate command facilities for conducting the battle. In Trek, no such separate facilities exist or are needed, so things are closer to the sailing-ship era again...
As for Troi or Crusher taking command of the ship in a "line of succession", I'd say it would never happen, quite regardless of their rank. That's not their job, they are not line officers - and indeed there would customarily be objections to giving a medical officer line officer authority, because such a person could theoretically declare everybody but him- or herself medically unfit and have his way in commanding the ship. No other officer would enjoy such unfair and dangerous powers.
Of course, Troi and Crusher are likely to have the basic training necessary for the menial job of watching over the bridge, just like, say, Ensign Kim or Ensign Chekov. So they can get assigned this duty when the top officers so decide, even though they in no way feature in the line of succession. Picard could assign Livingston the Fish such duty if he so wanted; but obviously, even Crusher is a better choice in "Descent".
Timo Saloniemi