1) Resident protection really NEVER leave something it can detect?
No, not NEVER, as that is a pretty strong word, implying 100% certainty.
But it is a decision I am comfortable with. In all the time I did scan with my installed AV in the last 3 years, I have only had one type of detection from an on-demand scan....false positives, and PUP detections for programs I knowingly installed. But I have had plenty of real-time, legit detections in those 3 years.
My personal view (and many tests [
] of Avast! over time support this notion) is that Avast! is firstly a
prevention anti-virus. Its best point being, it is really good from stopping you from getting infected to begin with. And if you do get infected, the resident protection is often still going to alert you (even if the malware itself is undetected) by its Network Shield.
I still scan, just not with Avast!, I use MBAM or SAS for scanning, as I feel that using a scanner with a separate definition database gives me the most benefit per minute spent scanning.
edit;
EDIT: Also has avast! blacklist synced with virustotal,mal0de database etc?
I'm not sure about that, if they draw off other sources, or if the blacklist is based solely on Avast! detection.