Interesting question and I think it's very important to be cautious and have all your bases covered for this exam. The WREB examiners are not very lenient, but they are reasonable. If the lesion looks good, I would use it. The reason they don't want to see periapical involvement on the tooth you're working on is because doing a DO on a periapically involved tooth is misdiagnosis and improper treatment. As long as #5 has a proximal contact, occlusion, etc. it should be fine. #3 needs an endo or extraction, and under ideal conditions, this would be done before treating #5. But the WREB examiners already tested your knowledge of treatment plan sequencing with the PATP exam, and quite frankly, very little about the WREB exam is ideal. The only other type of situation I can imagine #5 being rejected despite having proximal contact, occlusion, etc. is if every other tooth in the mouth is non-restorable and the patient needs complete dentures. Again this is improper treatment.
Good luck to you. The WREB exam is very challenging and will push you to your limits, but be confident and take it one step at a time, you'll do great.