I worked for an audit agency of the US Federal Government for 5 years. I was not an auditor although I occasionally participated in audits as a technical specialist. I also was reponsible to reviewing all audit reports.
You are an auditor. It is your job to point out what is being done incorrectly. People don't like that, so this kind of conflict is inherent in the your job. There will always be defensive organizational groups. You cannot soften the blow or you will stop being an effective auditor.
The first thing you need to do is to get a thick skin. You have to let any criticism of yourself and your report be interpreted by you as professional and not personal. You need to be prepared to defend your findings but not be concerned if the people who you send them to are unhappy. If they are doing their jobs incorrectly or illegally, it is your job to point that out. Of course they are going to be unhappy.
The next concern is to not let criticism make you decide not to present all the findings as strongly as you need to present them. It is NOT your job as an auditor to make people happy; it is your job to point out what they are doing incorrectly. You are an outside force for a reason, you need to stand outside the corporate politics of this. You will win some and you will lost some (yes even when you are right.) but what you can't do is stifle your voice because people might object.
You also need to present your findings dispassionately. Take great care to present only things backed up by facts (you should be tying everything in your report to a specific source anyway) and are not just your opinions. For instance, don't say these people are disorganized and stupid. Say they failed to do this task required by this law. They failed to perform this internal control required by this corporate policy. All costs that had receipts did not appear in the accounting system as required by the GAAP, etc. That is what I mean about the difference between facts and opinion. Let their management draw their own opinion about the relative competence of people.
And yes you need to allow them to comment on your findings. Again, simply send them to the people involved with a due date for scomments to be received. The email or letter should just be a simple. Here is the report. Please provide comments by 1 Jul 2015.
You need no discussion in what is being sent. The report should contain the facts you need to make your case.
I also wanted to add that they are likely behaving the way they are as a tactic to make you back down. Your report threatens their jobs. So they are trying to discredit you or make you feel uncomfotable enough to drop the findings. YOu need to pay hardball with people like this. Don't back down on anything that is valid. However, in the cocurse of the review, they may have valid points to refute what you say as the auditor frequently may not have seen all the information. Make sure to be scrupulous about evaluating their aruments on the merit of the argument and not how you feel about the people making the argument.
If you can make changes based on some of their counterpoints, then to the senior managers, who are the real audience for the report, you will appear to be the reasonable person. Don't fail to provide as much information as possible to refute any changes they request that you cannot find valid. However, including the objection with your refutaion again shows you in the better light than just ignoring it.
Remember every time you do an audit, the potential to cost people their jobs or send them to jail exists. You have to be ok with those results to do your job effectively. If they are committing fraud, they deserve to go to jail; it is not your fault for pointing out crimes they committed. If they are managing their organization so badly that the senior managers in the organization decide to relieve them of their duties; that too is not your fault. Your job is to point out problems; it is also to point out potential solutions. It is not to implement solutions. If someone is costing the company due to their poor management, it is not your fault that he or she loses his job. Be aware though that the more they have to fear from your report, the stronger they are going to be in fighting it. Talk to your boss about strategies for dealing with objections.
Make sure that what you did is not something that can be attacked on a factual basis (i.e., check your math for instance. I once found a 12 cent rounding error in an audit report that found $400,000 of cost savings. The activity we audited would have used that rounding error to discredit the whole thing. So check every fact and figure in your report meticulously (and do any calculations by hand as well as in Excel formulas). Check that values in the appendices match the values in the text, etc. Make sure you have backup documentation for every single number in the report.