Hello and welcome to you and June. I'm sorry you had to join our club but we're glad you found us so that we can help you as you continue along this path. I hope you don't mind I moved your post here but since you have a lot of pre-surgery questions this is a good place for them so that future members can learn too. I'll try to answer all of your questions as best as I can:

It seems that the location is somewhat rare.  Does anyone know if this location produced different results? Some locations I have been told can reduce longevity.

Our Jerry had his osteosarcoma tumor in the scapula. Many other dogs have too. I cannot tell you if there is documented evidence that location matters. I do know that I've read in places that the lower down the leg the greater chance of longer term survival, but Jerry's was in the scapula and he lived two years, with no chemo. Cancer basically does what it wants and you can never predict with 100% accuracy how things will turn out. All you can do is go with the flow, make educated decisions and learn how to Be More Dog .

Is it true that if we didn’t see any mets on her lungs 2.5 weeks ago she would likely have at least ~3m of pain free time with our family? Is the average truly 3-6m? 

There are no guarantees unfortunately. The 3-6 month range is with amputation alone. With chemo and now immunotherapy in many cases, dogs are living longer than ever, sometimes multiple years like Dexter. Some dogs' journeys follow the average prognosis, many others like our Jerry do not. In our experience watching members go through it here, there are many outliers that defy those numbers. So whatever you do, try not to focus on the numbers and instead focus on the fact that amputation buys quality, pain free time together. Remember, dogs don't read calendars and they don't come with an expiration date stamped on their butt. Nobody knows how much time we have, with or without cancer. 

And when it spreads is it truly less painful than her current scapula and leg?  We are trying to prepare and answer our children’s questions.

Absolutely. Bone cancer pain is the worst there is, ask any human who has been through it. 

When thinking about answering your kids' questions, you may want to read about Stacy and Griffin's recent experience with young relatives.

We do have size on our side and she has been adapting/reducing her use of the leg, even running without at times.  Do you think a handi-harness is necessary given she is 19lbs and we can easily pick her up?  She is a dog that doesn’t like to wear anything including a collar. She historically very able-bodied, lighting fast runner, lean and has major jumps.

Probably don't need one, especially if you don't have stairs inside your home and you can easily pick her up when necessary.

I hope this helps. Ask as many questions as you want to feel comfortable. We are here for you! Be sure to check out Jerry's Required Reading List for more helpful tips and stay tuned for feedback from others.