Regarding scan button behavior, this may be a bug on our side that will be fixed in the future. The intent is that both tapping and scanning perform identically (process template if in view).
Regarding your suggestions:
1. Some devices do not lend themselves to this behavior (MC67 has a resistive touchscreen, users may prefer to scan via scan button).
2. Automatic detection and processing is tricky, as the image must be focused and the user must have stopped moving. Currently, it is better to instead allow the user to determine what is a "good" image (if they do not know, training should be provided).
4. Using sound or vibration may confuse the user, as this is usually reserved for a successful scan/decode. We'll look into other ways of providing detection feedback.