I did some reading on this recently. In a quick cruise of my bookmarks I don't see the links (I can look again when less tired), but basically what I found was that studies show tree fruits do not put much of the copper and arsenic from treated wood into their fruits, although it shows up in other parts of the tree. The fruit is pretty safe. (The study was on particular varieties of apricots and apples, and the guess is that this is true for other fruits as well.) Vegetables that you eat the roots and leaves of, OTOH, are more problematic. Especially roots. Near houses and roads, lead is an issue. It is not recommended that you plant leafy vegetables there -- or, if you do, wash them very thoroughly. A lot of the lead is in surface dust on plant leaves, and on the dirt on root vegies, rather than in the tissues of the plants, so you can wash it off. You might consider raised beds... if you know the history of the dirt you would be importing.

In terms of human and plant health, it can't be any different than a vegetable bed adjacent to pavement made of asphalt (which is chemically synonomous to bitumen). I have one such bed, growing stuff well, and I haven't keeled over yet. At low temperatures, it's pretty inert stuff.

The manufacturer will have included instructions about drying/reapplication/finish on the product label. You will need to use the same type product when adding darker, but do not expect the darker to look like the store/etc. display, since you are coloring over another color.

We are located in Tennessee, near the Alabama line. Our weather is a mixed basket, a little bit of everything but not too harsh and it never lasts long.