Should I reveal structural issues to selling agent

YES! or get it fixed. It will come out no doubt in a building report anyway. You would need to be prepared to lose a sale or take a hit to the agreed price.

 

If concerned, get your own b&p done then give it to the agent. It may alert prospective purchasers to some issues but you have had an expert report undertaken.

 

If someone chooses to disregard due diligence, is it your responsibility?

Often things stay up until they fail

Plaster isn't structural.

 

Get some quotes, you'd be surprised how inexpensive a ceiling hanging beam can be.
Say $40/metre for an LVL beam, and with easy roof access the beam would be installed in a couple of hours. Shouldn't cost more than $500. Only issue would be access.
I'd rather fix it and sleep well, and not give purchaser any bargaining power.

 

I am going to go against the grain. From what you have said it is only a problem if someone is crawling around in the roof cavity, you are not making any claims that you could hold a rave in there. If it is fine, the roof isn't noticeable sagging or showing any other risks I would avoid touching it.

Old houses are difficult and you can end up chasing ghosts, yep you could throw in a hanging beam however it may actually pull the plaster or ceiling up or off its current and crack it, then you need to patch & paint.

You are selling an old house not advertising a potential second store loft to hold raves in. If you are concerned put a note on the edge of the roof cavity you have replaced stating 'this area has not been loaded tested and is original ceiling support, it is not intended to hold additional weight, do not go into this section'. Although saying that the chance of a building inspector getting up the cavity is low at best.