In order to avoid difficult situations, when we used to go to restaurants, we would always book for when the restaurant opened so we could choose the most secluded table possible (to avoid too much noise and distraction) and hopefully wouldn't have to wait too long for our meals as we were the first customers. We also pre-planned what we wanted to order to avoid delay and stressing Mum out over making a selection (we tended to have the same meal which also speed things up). We didn't linger either - normally just main meal and dessert then home for coffee.
We then had an episode where as a group of 4, we went to a lovely, very quiet, hotel to celebrate Mum and Dad's special wedding anniversary with champagne and a cream tea. Unfortunately Mum suddenly developed a foul mood when we sat down and refused to eat or drink very much (she gets very overly polite and sits there stiffly saying no thank you to everything with a glare on her face). The waitress was really bending over backwards to try to please her but Mum wouldn't bother to converse and kept trying to leave (threatening to walk back home although it was several miles away). It was so embarrassing, we were all having to shovel the food down as quickly as possible and Dad was fuming. It just felt like we were with a child who was showing off
This was about 4 years ago, the last time we went out for a meal as we saw that Mum's illness was progressing and she was too impatient to cope with restaurants (and it was no fun being on pins, worrying if Mum was going to kick off). So now we go to coffee shops for a drink and a cake during the week when it is quieter. That way if Mum has had enough after 10 minutes and is agitating to leave, it is easy to drink up and go. If we want a family meal, it is fish and chips delivered at home and if Mum doesn't want to participate then it's no big deal.