You are always safe if the virus is in the Chest and if you think that there are really virus then delete them all or if you think thats a false positive then wait for few weeks and scan them again or report to Avast Team
You have done the right thing, ‘first do no harm’ don’t delete, send virus to the chest and investigate.
There is no rush to delete anything from the chest, a protected area where it can do no harm. Anything that you send to the chest you should leave there for a few weeks. If after that time you have suffered no adverse effects from moving these to the chest, scan them again (inside the chest) and if they are still detected as viruses, delete them.
There is no need to delete those in the “System files” section of the Chest. These are system back-up files copied by avast in case of they are needed to repair important system files. They are not infected and should be left where they are in the Chest.