Coleoptera I don’t think you can replicate the interviews (and certainly the practise interviews school offered for my boys were nothing like their actual interviews) but I think finding people prepared to chat and challenge their ideas is easier. So for one son I asked a friend whose husband I knew worked at something vaguely similar to ds1 interests and has a reputation as a bit of a maverick to meet with him for coffee. This was someone ds didn’t know but who challenged his ideas and put him on the spot. Both sons also found work experience very good for talking to other adults who they didn’t really know and having thinking challenged.
I think you need to help them move away from the idea that their interviewers where somehow trying to thwart them and accept that these people will probably have behaved like that with all the candidates, and surely must have made some offers. They need to learn to deal with that sort of person, rather than hope the interviewers will change. Luckily they have plenty of time to fit in work experience before the next phase, and for you to put your feelers out in your network for anyone who might be suitable to talk with your sons. Remember you don’t just want nice people.
From my own sons experiences and anecdotes from friends who did or did not get places, being able to think on your feet seemed to be the deciding factor. Those who were brilliant but had very prepared answers didn’t do as well as those who worked with the interviewer and asked questions, made progress, were redirected in their thinking. The interviewers want to decide if they want to teach candidates for 3 years - they want to feel the person is listening and learning from their input.
As a previous poster alluded to I also wonder if the fact your twins have less than stella GCSEs and were then predicted exceptional A levels may have triggered alarm bells that the school were over predicting. Some schools seem more interested in being able to say they had a large number of Oxbridge offers, than if the students would be realistically be able to meet any offer made. Now that your dt have those exceptional A level grades that is one thing that can no longer be held against them.
I have heard people say that only listing Oxford/Cambridge on your application in time for early deadline, and then adding other universities 2-3 weeks later is a way to not let them know you are applying to Oxbridge - but I’ve no idea how true this is, but it might solve the long wait for Durham for example.
Finally I do know of a young man who failed to get an Oxford history offer but was successful with a post A level application to Cambridge.
Good luck to your boys - it has clearly been a stressful year for you all, but their amazing grades will hopefully give them the confidence to move on to whatever comes next.