Essentially, the two sentences express the same thing: people who do not have their belongings anymore will turn into miserable people. For most people they will be interchangeable. There is a slight difference in the emphasis.
Using the present perfect emphasizes on the result. It talks about people who have had their things taken before now, at any point in the past; it doesn't matter when exactly, what matters is that they do not have them now. The result is important.
Using the past simple simply talks about an action at a moment in the past. In this case I would think we are talking about one particular occasion when someone took their belongings because I do not see anything in the sentence that implies repetition.
Whether to use "have had" or "had" depends on the context. If you are talking about a time when many things happened, one of which was that people had their belongings taken, then I would use the past simple because there is one particular past moment we have in mind. If there isn't one, and we want to say: "people who do not have their belongings now", then I would choose the present perfect.
About the second pair, you are right in your assumptions. Let me expand the phrases. The first one is: "people who normally/usually smoke". the second one is: "people who have ever smoked before/in the past (whether regularly or even once)".
As emrys says, it is the since which is awkward here.
I think what the Guardian author means is that the tipping point was reached not in 2005 or at some specific point since 2005, but in the years since 2005. This would reflect what Murray and King said in the 2012 Nature paper: "In 2005, global production of regular crude oil reached about 72 million barrels per day. From then on, production capacity seems to have hit a ceiling at 75 million barrels per day."
But the past perfect is correct here; the author is referring to what was said a year ago by Murray and King, who were referring to the years between 2005 and 2012. The title of the Murray/King paper at that time was in the present perfect: "Climate policy: Oil's tipping point has passed".
Best Answer
There's no problem with this one -- it' a simple statement!
This one is fine, but is asking to be extended in some way, with something possibly in the present or the future. For example,
I have been to Mexico in the last year, but don't think it's too soon to return. or
I have been to Mexico in the last year, so I am familiar with the current political situation.
This one needs some extension, but with something also in the past. For example,
I’d been to China last year, but I was happy to return earlier this year. or
I’d been to China last year, so I knew what to expect when I returned recently.