With regard to your question concerning a backplate, no, the Origin does not have a backplate. You'll see the term used a lot on this board, and if you haven't been inducted into the BP/Wing club, you'll probably get confused.
A backPLATE is a formed piece of steel (or aluminum) with harness slots cut into it. A Halcyon backplate looks like
THIS. When you see "BP" used on this board, it's always used to mean "backplate". Other manufacturers make backplates, and they look relatively similar. On a backPLATE, a "wing" (air bladder) is sandwiched between the tank and the backplate. If you look around on the Halcyon site I linked above, you'll see how it fits together.
A backPACK is simply a device that holds a tank on the BC. The backPACK is joined to the body of the BC and whatever support mechanism the BC uses. The main differences in backpacks is whether it's a soft backpack or a hard backpack. A hard backpack is piece of plastic that has a concave indentation in it and a strap to hold the tank. A soft backpack does not have a rigid plastic piece on it.
The Origin does not have a backplate. It does, however, have a hard backpack. But the BC is padded for comfort.
Regarding the disparity between the published version of Rodales and the Internet version, the Internet review was conducted a couple years ago. It's possible that Mares has since changed the dump valves. Of course, if you're a cynic, the reason for the change in rating could be that Mares bought some more advertising or shmoozed the testers.
The question about buying online versus at the LDS has been discussed on multiple occasions. There are only about a dozen threads on the topic. Do a search and you'll find them. It is true, though, that the manufacturers will generally not honor warranties on "grey market" equipment. Vendors like Leisurepro, however, include their own warranties.
With regard to cost, one LDS I frequent has the standard "list price", a "sale price" and a "demo price." Strangely, everything is always on sale. And what's even stranger is that I special-ordered a new regulator and paid the "demo price" for it. I guess it was a "demo" regulator because they took it out of the box to show it to me. Between the "sale prices" and the "demo prices," the LDS is very close to (and in some cases cheaper than) online prices.
Hope this helps.