3. How can reincarnation be true when there are more people alive today than have ever lived before?
Since I was born in 1950, the world population has increased from about 2.5 billion to about 7 billion. It may be true that the sum total of people alive in the past is greater than the population on earth today, but at any one time, the current generation is larger than any previous generation (where a generation is defined as a group of people from birth to death). How many people of my age are currently alive? Let's arbitrarily assume it's 50%, or 1.25 billion. So where did the 7 billion souls of the present generation (which includes me) come from?
At some point, human beings appeared on earth. Only a few, but over time, the numbers in any given generation increased. On the face of it, if human beings are embodied souls, then not all of them can be embodied at the same time. There must have existed right from the outset at least 7 billion souls, maybe more. Either that, or souls are multifaceted and each facet gets to be incarnated; in the extreme, there's only the one soul that has, over time, accounted for all the incarnations we hypothesise today. We're all the same soul that has incarnated itself in billions of ways.
The maths of it is beyond my ability to compute, especially if we allow for some souls the possibility of reincarnating. I agree with Alex that I simply don't know, if reincarnation is a fact, how to account for it. I tend to believe in it because of the evidence, as well as the fact that it fits in with an evolutionary schema, but I could be wrong.
We have to distinguish between incarnation and reincarnation. I think we have convincing evidence for incarnation (i.e. human beings each have a soul or represent a facet of a soul), but the evidence for reincarnation isn't as compelling. Things like past-life memories don't firmly connect a live person with a dead one; it could be a case of some people having fairly arbitrary access to collective memory for a time after birth (past-life memories tend to disappear with maturation). Birth marks could be influenced by this, who knows?
The more one thinks about reincarnation, the more bewildering it becomes. It certainly isn't an open-and-shut case, but I don't think it's a ridiculous notion.