While it would seem prudent to ensure the executive summary includes important and significant findings and recommendations, and while it is not uncommon for busy people to only read the executive summary, I have not found any legal precedent for the report writer to be liable when the recipient has not read the report in its entireity, since:
(a) due to the requirement of executive summaries to be much shorter than the full document on which they are based it is never possible to include all the same information, and details which may seem important at the time the report is summarised may not be the same when decisions are made based on the report;
(b) executives are responsible for the decisions they make and this includes any due diligence checks and ensuring they have taken into account the risks involved etc - only partially reading a report which is of critical importance to a decision could be considered negligent on their part, whether the section read is an executive summary, conclusion or any other section;
(c) if all material facts had been disclosed in the report and you had reasonable grounds for believing your statement was true then it is unlikely a misrepresentation has been made;
(d) while a professional may be held liable for advice they have given, for example in a report (the legal terminology for this would be a representation), on which decisions were made leading to losses or other negative consequences, there would not normally be justification unless it can be shown or claimed that information was withheld, misrepresented (for example an assumption or opinion should be declared as such, though in court an opinion may be treated as fact if made by a professional with specialist knowledge on the topic) or inaccurate (the report writer should check that information is accurate at the time the report is delivered not just when it was written as material facts can change over time).
While the report writer will likely have carried out their professional responsibilities with due care and diligence in accordance with the contractual requirements whilst exercising professional judgement at all times, it is important to ensure any professional judgements if made can be aligned to industry best practice or standards or have a documented process for how the judgement was reached including the knowledge gathering and analysis, the assessment of relevant guidance and the process for making, addressing and challenging that judgement.
I recommend you keep a written record or log of communications as this can help to provide evidence to defend a claim if later required, and if your business is issuing professional advice such as this it should really have professional indemnity insurance to cover the legal costs of advice and defending against claims relating to advice given.
Disclaimer: I am not a law professional and the above should not be considered as professional legal advice!