There's a big difference between "settled" and "set in stone".
"Settled" science can be challenged, but you just can't waltz into a field and say, "I have this data which proves that decades (or even centuries) of research are entirely wrong." You have to start with narrow claims and then gradually broaden them. You attack scientific consensus by patiently tugging at loose ends until the whole fabric of consensus starts to unravel.
Science is, in fact, open to the possibility of perpetual motion or intelligent deign. It just doesn't make it as quick or easy as some people might like to make such ideas a new scientific consensus. The value of scientific consensus largely lies in that it must be hard-won.
So to answer the summary's question, of course science can be "settled", but settled science can always be overturned. A "settled" hypothesis is merely one so well-supported that the burden of proof lies with its would-be rebutters.