Option 2 - perhaps whoever it may have been didn't show up. A tentative arrangement is still a possibility.
Option 3 - DIL (daughter-in-law) - ok, possibly not likely if the loo is "basic".
Another possible option is if they happened to serve some kind of drink / snack that wasn't available at the Dolphin.
Or could the Dolphin have been noisy / crowded, hence a possible decision to leave and move somewhere else for a quieter moment (Option 1 ish)?
My main point is that there could be any number of reasons. If it had been considered important, the PJ should have asked.
OK, Kelly's did not serve any kind of food beyond peanuts and crisps, so I cannot think what might be the option there.
Could the Dolphin have been crowded or noisy. From memory, the Dolphin was neither empty nor packed that night. I don't think crowded is an option.
If you check my blog, you will see that while back, we ate out at Paulo's (recommended). During the course of the evening, a number of groups came in. I like that. I hate eating in empty restaurants. That tells me I am in a place that people do not rate highly.
So, the evening progressed, Paulo's got a bit more lively. One group came in, of four, with a particular woman in tow. She appeared to think that everything should be shouted. As everyone else could only make themselves be heard by shouting, the place got quite noisy, and not to my liking.
So quite possibly the Dolphin got noisy. Why not just head for base? Early flight, pregnant, unwell woman.
May I address your main point?
Based on my experience of Luz (and my understanding of the case files), it appears to me that a considerable number of officers were deployed on the case who had limited understanding of the case, and even less of Luz.
I am not having a dig at the PJ. Exactly the same 'error' has occurred with numerous investigative journalists. Exactly the same 'error' has occurred with Operation Grange. I suppose I must stick an IMO here.
First, from the statements it looks as if the PJ interviewed a single barmaid from Kelly's. Memory says Sep 2007. If my memory serves me well, that is around 5 months after the incident. So, 5 months on, what should we expect this barmaid to remember of a non-remarkable night. Probably nothing.
Second, from the statements, it looks as if the bar maid was asked if the Smiths had been in that night. As I have said, I am not a pub frequenter, so please make up your own mind. Do customers get known by their surname in pubs? Is that how it works? Good evening, Mr Smith, would you like your usual, and how about you Mr Smith, what would you like tonight?
Third, the sequence of events seems to be bar maid interview first, followed by till roll copying second. That order may be extremely important. If the PJ had a look at the till roll first, they might have seen that the pattern suggests strongly that Kelly's was very, very quiet that night. Now they didn't have knowledge of the fact that Kelly's was a popular bar, and so a very quiet night was an oddity, to say the least.
What they would have got though, is that on a very quiet night, a group of 4 adults and 5 children would have been very memorable. Not 'the Smiths', but just a large party on a quiet night.
Fourth, the PJ (as per many others) are unaware how Luz works. Fernando's (aka café bar Calhete) - single person duty. The Bull - two people on duty. Kelly's - the norm is two people on duty. If so, we are missing the evidence of one bar steward who was on duty that night. The PJ seemingly had no idea that Kelly's usually has 2.
To return to your point. Should the PJ have asked why the group moved from the Dolphin to Kelly's. IMO yes. Should various PIs have asked the same question? IMO yes. Should OG have asked the same question? IMO yes. Should any investigative journalist worth his salt have asked this question? IMO yes.
I happen to think that as of Valentine's Day, 2016, this is one of the top 10 questions in this case. Wouldn't it be nice to get an answer?