Hi,
1/ AFAIK no, there is cryptmount used for userspace, so Avast! is scanning files only, no decryption is needed; but off course files shield could drain the battery when new files are encrypted via system and scanned in the same time.
2/ Yes, encryption is battery consuming operation, but is done once for all mounts and (if set) decrypt is used after restart. Some of the battery issues are solved in last android update.