1. The purpose is to see if it might find something that may have been downloaded yet undetected by avast, there is no certainty that will happen. It is a multi application approach to security by combining compatible applications to improve overall detection rates, no single application will provide 100% protection.
2. When you say you clicked on the link for MBAM, how (?) exactly what did you do ?
You should be right clicking and selecting save as, etc. as per the instructions to download to your hard disk.
3. For the reasons mentioned in 1. above, these are specialist anti-spyware/malware, where avast incorporates anti-spyware into the regular scanning, the others do scan the registry for spyware or suspect entries, avast only scans your HDD and if spyware is found then it would check for associated registry entries. There is a subtle difference as some registry entries can be modified to lower security policy, etc. preparing the way for malware when malware may not actually present or hidden/undetected.
4. As the instructions on sending to virustotal (VT) said, you can't send them directly from the chest, avast would block that (0 byte size). From the outside of the chest you can't see the original file name avast changes that and the files are encrypted, this is all part of ensuring they can do no harm in the chest.
So they have to be exported to a temporary folder that you have created and excluded (as per the instructions) so they can be uploaded to VT.
5. If you read the link I have on the fujack info there can be much more to this complex worm relating to the registry and that is why I suggested the specialist tools for the job.
