I appreciate the detail in your response and being 'prepared' is never a bad thing but can't help thinking it is slightly alarmist in these circumstances. I would hope UKBA are not on the look out for retired couples in their 60s from the USA who want to visit their grandchildren. Would be an awful waste of limited resources when there are much bigger problems afoot.
Perhaps it might be a little alarmist, but then you can never predict who will get bounced and who will be allowed into the UK. It's always risky for anyone to try to visit for a long period of time (i.e. more than a few weeks), regardless of their age or circumstances, so I think it's something they will need to consider.
In the case of a retired couple in their 60s from the US, the IOs may be on the look out for people who might try to retire in the UK without a valid visa in order to get free healthcare on the NHS as opposed to having to pay for medical care in the US... especially if they have children and potential/young grandchildren already in the UK, which would be all the more reason for them to wish to retire in the UK.
As there have been a few members of the forum who have asked if they can bring elderly US parents to live with them in the UK and due to the fact that elderly dependent relative visa is extremely difficult to qualify for, I suppose it's not an unreasonable assumption for an IO to make.
If you have an open ended ticket because you are not sure when you will come back, does that suddenly mean you are never going home? I would like to think the UKBA have more intelligence and I am sure they do. The real criminals don't get open ended tickets.
An open-ended ticket with no definite return date booked is going to be a big red flag to the IO. The most important piece of evidence they will need to show is a return ticket for a specified date and it is usually the first thing the IO will ask for, before any other evidence. If you don't have one, it's unlikely they will let you in. (I have a visitor visa for the US and the last 3 times I have entered (each trip was only 2 weeks), I have been asked for evidence of my return ticket to the UK).
Also if you are saying everyone gets 6 months in ‘theory’ even if someone initially intend to stay for 2 weeks it would surely mean that the length of your stay (assuming it is intended to be below 6 months max) isn’t as important as other factors. If you decided you suddenly wanted to stay for 3 months after telling UKBA ‘two weeks’ there is no one to ask you any questions at that point and the people who 'vanish' in the country are likely not going to say they are staying a long time at the gates. Those that are honest and say ‘actually I am staying for 2-4 months with my family’ you would like to think get credit for that. But maybe not.
Well, it's all about intention at the time you enter the UK.
If you intend to stay for 2 weeks when you enter the UK and you tell the IO that (and have a return ticket), but then the day before you are due to fly home, you decide to stay a bit longer, that is not a problem (as long as you don't stay more than 6 months), because you were honest with immigration when you entered the UK and your plans genuinely changed at the last minute.
However, if you tell the IO that you are going to stay for 2 weeks, but you fully intend to change your ticket and stay for 6 months instead, then that is considered using deception to enter the UK and if the IO discovers this (through further questioning), then they can and will refuse you entry and you can be banned from entering the UK for 10 years.
So, you need to be honest with the IO - if you intend to stay for 2-4 months, then you need to tell them that (although you will need to have a definite planned return date and a ticket stating this date).
However, because 2-4 months is much longer than the regular tourist normally stays (especially considering that US workers generally don't get much annual vacation time), you will need to be prepared to show extra evidence that you are in fact a genuine visitor and that you really do intend to leave after those 3-4 months... the return ticket with a specific date is the most important, then your finances and ties to the US to show you must return home after those 3-4 months.