So talking you out of accepting indemnity insurance is something conveyancers do with first time buyers, I have had it in the past.
What indemnity insurance gives you is protection if the council sue you. What it doesn't do is protect you from shoddy workmanship.
If you have lived in the house for several years and not seen issues with those two areas, and especially if they were done a long time ago, chances are, they won't cause you an issue.
That leaves you with the council finding out and suing, which is what the indemnity is for (you can't then ever apply for sign off, it invalidates the indemnity). Chances of that happening? Slim to none. Don't ever tell anyone they don't have sign off.
What conveyancers don't actually relay to FTBs is that most older, experienced buyers won't really care. They will have seen many houses without sign offs, and an experienced eye will tell them whether the changes are likely to cause structural or financial problems.
So, it's entirely your call now stern you want to be. It might make the house a little bit harder to sell in the future, but there are also many people out there who just won't care if they like the house enough.
Your sellers won't budge because they know many other buyers simply won't care, and they can't be bothered arguing with you anymore.