Engineered wood Floor

Depending on what the engineered flooring is, it might, but more likely it won't--at least in a way you want it to. At best it might simply flex a little, at worst it will crack the laminate, in the middle it'll not only flex but squeak besides.

Need to have a level, smooth subfloor. Consult the manufacturer's instructions for how to lay the product.

The board will not settle over time. It may bend and flex, but it is going to pop back up just about forever. I'd put a shim of some sort or replace that one or two boards that dip. 10mm is fairly deep. Bondo may do hte job.

A shame to have to cover an original pine floor. Sounds like it's in keeping with the design of the place. We pulled up the edge of an old carpet in our 1908 Portland home and found what looked like fir plank flooring. My wife was ecstatic. Another few feet of carpet pulling and we discovered the contractor who had flipped the place had taken the nicest fir out and replaced it with a hodgepodge of flooring materials. He probably has a lovely floor now.