issues regarding student research work in an undergraduate level
I think you have built a counterproductive structure into the course and you should probably change it, perhaps radically. 

But there are some lesser things that you could, perhaps, do to make the problem go away. Maybe this isn't possible, but your staff should discuss these and all other options. 

First, is that you can decouple the "advice" given by the reviewers from the grading/marking of the work. The actual marking and assigning of grades should be someone with an overall perspective on the course and its students, not the parochial views of some specialist, no matter how skilled. 

The second thing is to have a session before the work goes out to the reviewers in which you are clear to the students about the marking (if you can get it changed) but also that they are likely to get weird, even hostile reviews. The students should be clear that while they need to be respectful and to consider any proposed changes or extensions that such things won't affect their marks. But of course, if project revision is part of the exercise the students will need to show that they have considered suggestions even when they must be rejected. 

Another thing you might be able to do is to give the reviewers some training. Perhaps they don't understand the requirements at the level the students are at - undergraduate. Perhaps they don't understand their role - helping, not abusing, students. You can probably do this with written material given to potential advisors at the time of selection. A face to face meeting of all reviewers would be preferable if possible. That way you get feedback on the attitudes of the individuals. If you can fiddle with the grading scheme, you can make it clear to the reviewers that while their reviews will be considered in grading, they won't be definitive. 

Finally, you may need to do a better job of matching students and reviewers as well as rejecting reviewers who have behaved badly in the past.