How do we invite people who are not on WhatsApp?
Print the invitation with a code on it. Scanning that opens the same page the rest of the family has, with the current times and the reply button. For anyone who would rather telephone, take the answer by phone and keep one list of yeses so the count is not split.
Should the invitation mention parking and steps?
Yes, generously. Say what the ground is like, where a car can stop, and whether someone will be at the door to help. Older guests decide whether they can come based on exactly these details, and nobody enjoys having to ask for them.
Can relatives add messages, or only the hosts?
The wishes section is for guests. They write, you approve before anything appears, and the notes stay with the page afterwards. It is a wall for the day rather than an open feed, which is what makes it safe for a family this size.
Do we need a separate invitation for the coffee in the morning and the lunch?
Not if the day is in the running order. A page can hold a multi-part schedule, and even a second day, so people can come for morning coffee and return for lunch, and you still know which headcount belongs to which table.
What happens if the venue changes after the cards are printed?
You change the page. The code on the card opens whatever is true now, which is the main reason to put a link on paper in the first place. Drafts stay private, so editing after sending cannot confuse anybody.