I'm not sure if they are correct about you having to have an online tree. When I look at the DNA results there are a number of matches where the people don't have an online tree and more than a number where the tree is private.
I think what they meant to say was that it is more effective if you have a public online tree. That way any potential matches can take a quick look and move on.
As for sync'ing only one way. That is what I do. I don't make changes in Ancestry, only locally in Family Tree Maker. You, yourself, have to be disciplined about it. The only exceptions I make are the occasional image that I will attach on ancestry and then sync to have the image locally attached.
You could, of course, just upload a tree to Ancestry without syncing.