I think the latter two questions may lend themselves to order of magnitude estimation approaches, but I don't think the first one does.
Q1: Why is our body temperature around 30 °C?
The first clue is in the original definition of the Celsius scale: 0 °C the freezing point of water and 100 °C the boiling point.
Life as we know it relies on chemistry and a solvent allows chemistry to happen on a reasonable time frame, as molecules can dissolve, diffuse, react. The solvent that makes life possible on Earth is liquid water, and at the pressure that Earth's atmosphere ended up with, it is liquid between 0 - 100 °C. As we're 70% water and we'd like to keep it from freezing/boiling in our bodies, that gives a general range for where our body temperature would be. Note it's just a general range-- there are plenty of animals with different body temperatures, including some fish with antifreeze proteins which allows them to go a bit below 0 °C.
Q2: How can you estimate human pregnancy period & lifespan from order of magnitude estimation?
The more information you know the more accurate your estimation. For example, say you know cell division in humans takes 24 hours. Then pregnancy probably wouldn't take thousands of years and would certainly not be shorter than 24 hours. Depending on what kind of information you have access to, you can improve the bounds.
Estimating human lifetime is a bit stickier because there is no genuine answer to "how long does a human live"... some humans only survive a few days or less and it's not entirely clear that future humans won't live thousands of years.
I recommend looking up Fermi problems. Seeing a number of examples can help you understand this way of thinking and where it is appropriate.