To answer your questions,
1) Yes, it would be very likely would play a part in your application result. I'm not sure how strict are they with the family background screening but even if hypothetically speaking about your family has criminal background and you made it in, I think your opportunities to go for operations will be very limited and by extension, your chances of getting promoted. Just give it a shot and see how it plays out.
2) Yes, DES's SA does include an interview. Please refer to this guide I've quoted below.
You don't need to photocopy the guidelines side of your N Level cert.
If you have documents that come in two separate pages, photocopy each page on separate pieces of paper and staple them together; don't copy them back to back on 1 page, they will ask you to re-print. Speaking from experience.
Hope this answers your questions! If you got further questions, feel free to reply to the thread or DM me.
Good luck!