Search the archives here for information that will be helpful to answer many of your questions.

There are several brands of waterbourne polyurethane finishes in the Bona line.

To answer one of your questions: you do not double-stain a wood floor...doing so can lead to peeling of any film-forming layer placed over it.

Resistance to indentation is a function of the wood hardness. A harder finish will not protect your floor from dog nail indentations but after it cures it might be more resistant to surface scratching and scuffing.
Whenever you move furniture on a urethane finished floor, you always pick it up. Regardless of the type of finish, sliding a piece of furniture can scratch the finish.

I'm a proponent of oil modified polyurethane due to the ease of application, ease of application, appearance, durability and value.
Many people claim the waterborne urethane wears better in testing but real world experience shows me that the oil based finish holds up better. A representative who worked for a maker of both types of finish, told me at a seminar that oil-modified polyurethane is more durable than the best waterborne urethane.
That said, the waterborne is in widespread use and is sufficient for most applications. Some finishers and customers prefer the waterborne finish for environmental reasons and for drying time.
You should only need one coat of stain when you're covering it with urethane.

Thank you glenn & JFC. I appreciate your knowledge. I've had wax on my floors from when I 1st bought the house yrs ago & you can see a path of raw wood where I've been walking from one room to another now. It's very rare for my floors to get scratched when I move anything. It's actually never happened & I'd like to keep it that way. I laid, stained, & poly'd some wood floors in a bathroom (I know crazy, but it looks awesome!) years ago & have not had any trouble w/ scratching on that. I couldn't tell you if it was an oil poly or some other kind, but whatever I used has been very good. It still looks pretty good 8 years later. So, I want all of my other floors not to scratch like those, as much as possible. I've had a couple people tell me the waterborne just isn't much protection.

JFC, what is an oil-modified poly? Maybe I should suggest he use that.

Don't be afraid to specify a finish, such as Bona Traffic or an equivalent. Durability of quality waterborne polyurethanes are proven and can be found in service in commercial venues, such as restaurants, boutiques and museums. But, there are 'cheap' waterborne finishes and those you want to avoid.

If you can find a finisher who still uses oil-modified, and the product is still available in your area, I suppose you could use that. Just remember, that with solvent-based finishes the cure time is much longer than with waterbourne polyurethanes. Your research will give you the knowledge you'll need to make the right decision for you.