Eh, if you took away the birth encouragement and the one-child policy, it works out to be about the same. In 1950, the population of India was 350 million, while China was 555 million.
I don't think so. China's birth rate was so high in the 1950s, '60s and early '70s that the population has continued to grow since then, even under the one-child policy, due to simple demographic momentum.
If you slow the population growth back in the 1950s, it would have most likely halted altogether by now, as in the case of Japan. This does not require adoption of the one-child policy. In the 1970s, China reversed course and launched a campaign to encourage families to reduce their number of children, on a purely voluntary basis. This worked well: China's fertility rate was cut in half between 1970 and 1980. By that point it was approaching the (low) rates of its East Asian neighbors, but the government decided to go a step further anyway and implemented the one-child policy. Even if it had not done so, there is reason to believe that China would have sub-replacement fertility anyway, given the way it was trending, and especially with the urbanization of the population that came after the 1970s.
If China had adopted the voluntary campaign in the 1950s (instead of encouraging the opposite), it would probably have somewhere between 100 and 200 million fewer children born over the next two decades, and in turn they'd not have produced all the children they have. We'd probably end up with a Chinese population of around 900 million or so today.
Another possibility: have the Kuomintang win the Civil War, which probably leads to a similar population trend. China's economy would have developed and urbanized much sooner and it would probably have had very low birth rates for awhile now, like Taiwan does.