As I have a different view on just about every question, I'll chime in as well ^^
1. I have no personal experience (I've only had buttons for nearly 3 years, the first ones still live - to date I've never had an adult die in my care) but from what I've read males usually live for 6 years and females for about 4.
2. Game bird feed, yes.
They can most likely survive on budgie feed, depending on their age and what kind of supplements they get, but according to the most cynical source I've found (they suggest an appropriate cage size of 1 sqft for a pair) they need at least 19% protein. Protein is expensive, so I'm pretty sure if that source could have earned a cent more by feeding them less protein, they would've done so.
Also I've seen birds raised on less protein (don't know exactly how much) and the ones I saw were just about half the size of mine and their feathers looked horrible.
Apart from that, the first buttons I got are living in a budgie aviary. They eat what the budgies drop and prefer this above their gamebird feed. 3 of the 4 birds are plucked bare on their backs. I can't say fore sure this is because of the feed, but it's a common theory that too little protein causes feather picking.
3. They are flock birds and can die from stress if housed alone.
As long as there are no females around, two males will usually get along just fine. Add females and you have trouble. I do bachelor groups all the time when I have chicks that are nearing maturity and waiting to be sold and I very rarely have issues. However I'm not sure I've ever had one of these groups for more than 6 months, so I can't say for sure no issues would have emerged later, but I've also read about people on here keeping two males together seemingly harmoniously. They'll usually be breeding each other and of course it's not really natural for a male to be bred, but as long as there is no chasing around in order to breed and the one being bred just lets it happen, I don't think it's an issue.
As a comment on trios being better than pairs, I've tried trios several times and I usually end up getting nothing but trouble - they might be fine at first, but eventually one hen just can't leave the other alone. I even tried it with two sisters that were raised together. Added a roo, no issues for months - then spring hormones came and the chasing began. I put them in an aviary to give the one being chased room to get away and since then, they've actually coexisted nicely, raised chicks together and such, but even though the cage was pretty big (2 by 5 feet) it just didn't work.
Pairs usually do fine - just don't cram them in a tiny cage, give the hen room to move away if she doesn't want to be bred.
4. Grit is needed for digesting bird seeds, but not for digesting game bird feed.
5. Yes, you can, but usually plain water is fine. People sometimes use electrolytes for sick birds or as an occasional boost.
6. Coturnix quail are significantly better as pets. Much more calm. Also poops more, and make bigger poops though.
Now, as final notes..
You mention liking the sound of crowing. In my experience, happy buttons rarely do that. If you keep two males together, you will likely get a lot of crowing as they call for a mate, but the ones I keep in pairs might crow 1-2 times a day, sometimes not at all and only on rare occasions significantly more than that.
Beware of the bar size in a guinea pig cage. A button might be able to squeeze out.
I would not allow front yard foraging - at least not without a cage on top of the bird. It can fly, and significantly better than most people think. I once had one fly over the roof of a house. And unless it's very tame, I think the stress from being moved around by a human surpasses the benefits of forage time.
