We live in an imperfect world and councils will make mistakes. And there are always boderline cases where it is hard to know what is the lawful route. But those are not reasons for failing to comply with the law that governs local councils.
An approach that I find helpful is to consider whether something breaches the principle that the legislation aims to embody. So, for example, publishing agendas with proper notice is about enabling electors to make representations to the council if they are concerned about a decision that may be made. If something breaches this principle, then it is wrong. If nothing much was lost, then maybe it isn't worth worrying about.
If you are aware of something being wrong and consider it important, perhaps you could insist on your concern being minuted. In a serious case, this could be important in ensuring that you do not become personally liable for a recklessly unlawful decision.
There is plenty of information available, it should be possible for councils to operate within the law at least most of the time.