On iPhone, can I tell if group text messages were delivered?
No, sorry. The reason being is that it saying delivered would have to guarantee that it was delivered/read by each phone and one message would not be able to do that. Maybe Apple will add a more robust system in the future though.
You are correct, there are no delivery or read receipts for group messages (regardless of the size). Even when all participants are using iMessage, it will not display either receipt.
Note: Whatsapp does provide for this. Tap and hold a message. Tap "info". At the bottom, it will show who read the message.