Yes, you should stay provided you have a good boss and you are remunerated fairly based on market rates. Otherwise, it is not worth it. At the same time, you should also go for your studies, preferably part time in NUS, if it is feasible.
Whether it is a ponzi scheme, to me, it is not important. What is crucial is you need to know what they are doing with your money. The insurance companies are investing your money to generate higher returns so that they could pay out the money during a claim and still makes a decent profit. That is why AIG (Parent company of AIA) runs into financial trouble and needs to be bailed out during the 2008 financial crisis. Just my humble opinion.
Yes, it is better to pay more for private plan when you are young as the premium are relatively cheaper. However, do note that the premium would get more and more expensive when you are older, especially around 41 years old. This will only get more expensive as the medical costs in Singapore is escalating.
No. It is not guaranteed. The 3-4% pa is merely to entice you to buy the policy. Only the reversionary bonuses declared every year is guaranteed. Do note the reversionary bonuses declared is less than 3-4% because of insurance companies' practice of smoothing the bonuses. That being said, I am not saying insurance is not important. Insurance is in fact very important. But you need to buy the right type of insurance. Just my humble opinion.