We've towed the Edge locally (California) for a while and are now halfway through an 8500+ mile trip around the country.
I also had the same concerns as you regarding the 5000lb bubble, but two things: 1) It's not over 5000lbs. 2) In our experience, it tows like a dream. Our PC is a 2552, sort of the middle of the line. We gone over the Sierras, the Rockies, the Berkshires, and the Catskills and there have only been a handful of instances where I knew the Edge was back there and dropping our speed. Right now I think the only place I wouldn't tow it is in the hills of San Francisco! Also, we store some stuff in the Edge while being towed....not really heavy stuff, but beach gear, some chairs, fishing equipment, stuff we've bought and need to mail home, etc.
I'm not familiar with the ready brake, but you mention possibly needing to lower the tow to be more level with the rig. On this subject, I'd refer you to Ron and others who managed to get it in my head to ensure that the towed vehicle and rig equipment are within a couple inches of each other. I have no experience with mine being out of sync more than that, as mine is almost totally level.
Regarding the Stay-In-Play....I have no experience with anything else, but I can tell you that after the initial installation I haven't worried or thought about it...it works exceptionally well. One thing to bear in mind though....apparently, the company suggests that a little LED be installed above the dash in the toad so you can see when the brake is applied by using your backup camera. Um....we didn't do that. Instead our mechanic ran the little led to our PC dash (in the little coin holder) and I can see it every time the brake is applied. The Stay-In-Play is proportional and progressive. It requires both deceleration AND coach brake light to be activated. Step on the brake a little and you get a little...step on it hard and it BRAKES!
I must say it was a bit unnerving to have the tow gear installed on the Edge. I mean it's a $40k car and they had to do a bit of grill renovation to get the baseplate on, but it looks pretty good especially when you remove the easily removable tow 'tabs'
I've never had the problems that Paul has with the keyless ignition. But I may not be carrying everything he does in his pocket! I figure if my wife has 30 or so handbags, then she can carry everything except the car remote!