I don't agree.
At that point, Troi had not yet completed her bridge officer's exam (which we would later see in "Thine Own Self"). Thus, in "Disaster" she would not be qualified to stand watch on the bridge, not even in an emergency situation.
Realistically, Ro Laren should have been in command, since she was an officer on the command track.
A senior officer is still a senior officer unless someone of an even higher authority places someone else in command. As it was an emergency situation, the most senior officer is in charge. However, as Troi did correctly, she sought the advice and knowledge from those who did have the proper training. Even Chief O'Brian said this.
There are a few different things that could change that, say if they Enterprise had been under attack or something.
Even a senior officer without training for some situations, is still a senior officer and they may posses knowledge that the junior officers do not.
There are however, always situations when the rules are called in to question. You are correct, at that time Troi was not Bridge Officer qualified, and Ro could have brought that up. Perhaps if the emergency was caused by different situations then it could have unfolded differently.
But also remember, whatever track Ro was on, at that point, she was still only an Ensign. And despite Troi only being a Councillor, she was still a Lieutenant Commander. That is too large of a gap in rank. Again, perhaps if Ro was a little higher in rank. Ro may know lots about procedures and tactical command knowledge for some things, but a LtCdr, even a Councillor, still has to know things to get to that rank. Senior Officer most often have knowledge of things that the Junior Officers simply do not. And rank still needs to be earned regardless of your career path.
I think what most people are getting confused with is what and when Ro could take charge for a situation as that. IF Troi had begun to make poor judgement calls, while making things worse, of course yes, they could have the right to take over command. But again, as I keep saying, Troi did not do that. She acted accordingly and properly for her situation. She may not have been a Bridge Officer, but she was not serving as a Bridge Officer at the time, she was just there. And the serving Bridge Officers were dead. If the Bridge Officer on Watch/Command had not died, then No, Troi would not have assumed Command. But in that particular situation, she was there. Its how it happened. After the Officer in Command had been killed, there was no one who had appointed seniority, therefore, authority fell to the most senior officer that was there. Not ideal, but thats not it went down.
During the conversation about the state of this ship between Troi, Ro, and O'Brian, both Ro and O'Brian argued over separating the saucer section to save half the ship. Ro stated there was no evidence that anyone was alive, O'Brian that there was no proof they were dead either. Both were right, and Troi was the one to make the decision to do everything to help everyone. Even at the risk of the ships destruction. She wanted to give everyone a chance. And she came up with the idea to route power to engineering to try to get someones attention. Starfleet doesn't just give up and give up on saving the lives of others at the risk to their own. In fact, this is an inherent risk Starfleet personnel take when they sign up. Yes you could argue that then the crew would be willing to accept death to save half the ship. But I am fairly certain that trying to save lives if preferable to abandoning them. And Lieutenant Commander Troi was aware of this.
There was a point when the containment field almost collapsed, and Ro pointed it out and told her it was time to leave. Again, Ro was technically not wrong. But it was not her call.
Think of it this way... if every time there was a similar situation and some Junior Officer thought that they knew more then a more senior ranking officer because of their career path, then there would be far too many instances of mutiny. Any officer with poor judgment who thinks they are correct would be constantly challenging every senior officer and things would not work out. And this is exactly why Rank Seniority is a thing. I will also point out again that by the time an Officer is a full Lieutenant, they should know to ask for help or advice from other officers in situations that they may not have as much experience in. A good officer will know their strengths and weaknesses, and not afraid to seek help or advice where they lack. This includes good junior officers who will understand that no matter how much they know, they can still learn something from more senior officers. And I feel even Ro knew this. Although she voiced her opinions and thought she was correct, in the end, she still listened to Lieutenant Commander Troi.