Well, I dare say the memory scan is there for "historical reasons". While I'm not saying it cannot detect something the ordinary scan wouldn't (in the case when a sample, itself detected in the clean form, is packed or encrypted by something which the engine unpackers are not able unpack or emulate), I'd say something like that won't be very common (because avast!'s signatures are mainly file-oriented). Besides, if something is found in memory, it's probably too late anyway - the malware is already active.
If you tell me how exactly the Malwarebytes virus database is detected (what exactly the results show), I might be able to tell you a special mask to put into the list of exclusions for that particular scan - to suppress the detection.
Regarding the point 2... it's not there, because it wasn't implemented (yet?)
It's not that simple as it might seem, and it doesn't really have a high priority at the moment.