1) To my surprise, the packaging says it will work only on the vertex area. Is it true?
It was clinically tested on the crown/vertex area and so they can only state it as working in this area.
The realty is that some people can get some benefit on the hairline. It certainly helped mine to some extent, although the area was too far gone.
2) Assuming it works and the transplanted hair gets thicker, will it start thinning again should I stop using rogaine?
Only the native, DHT affected follicles would thin again. After prolonged use it is usually stated that after 6 months of discontinuation you will have lost all the benefits.
I'm not sure it would have much help on DHT resistant hair, other than to make it grow quicker, possibly.