imessage for some contacts and sms for others iPhone

She needs to go into Settings, Messages and turn off "Send As SMS". That way if the recipient is someone that can receive iMessage and it isn't available it will not send it as an SMS. That is the purpose of this setting. It only impacts those you iMessage with. With this setting off you can still send SMS to those individuals you normally send SMS to.

To exchange iMessages doesn't require being connected to an available wi-fi network. Exchanging iMessages requires an internet connection no different from exchanging email. This can be when either party is connected to an available wi-fi network or when connected to the cellular data or internet network with their cell phone carrier.

If your daughter has iMessage enabled and has internet access via an available wi-fi network or with her carrier's cellular data or internet network and the same for your wife, a message sent by your wife should be sent as an iMessage unless there is a problem with the iMessage servers for either area.

As provided, your wife can turn Send as SMS off for her iMessage setting on her iPhone.

One other question then.  Since daughter can receive SMS messages (but prefers not to), and wife can send SMS messages to non-iphone holders, how to prevent wife from accidentaly sending an SMS to daughter?  The answer seems to be to specify daughter's iMessage account by Apple ID instead of phone number?  Or does it not matter -- the phone knows she is an iMessage contact and won't allow an SMS message?

Since daughter can receive SMS messages (but prefers not to), and wife can send SMS messages to non-iphone holders, how to prevent wife from accidentaly sending an SMS to daughter?

Already answered.

She needs to go into Settings, Messages and turn off "Send As SMS". That way if the recipient is someone that can receive iMessage and it isn't available it will not send it as an SMS. That is the purpose of this setting. It only impacts those you iMessage with. With this setting off you can still send SMS to those individuals you normally send SMS to.