I carry a 9mm xdm compact IWB using a galco kingtuc and I'm very happy with it. Many will say go with a crossbreed as it's "the best", but truthfully they're all pretty much kydex and leather. Unless somebody really screws up the fit I don't see much difference. I opted out of the crossbreed because they mix religious views with business products and I disagree with that practice. Other than that I'm sure it's a fine product and you wouldn't go wrong if you chose that way.
Plenty of folks will advise against your wife carrying in the small of her back due to potential spinal injuries in event of a fall. You may view it as remote, but you never know. I took a spill in my driveway this weekend dismounting the 4 wheeler in snowy conditions. Both feet went right out and I went down hard. The only thing stopping me from going down on my back was bouncing my head off my bike which was kind enough to intervene on my behalf. Instead I just busted my arse. I don't carry small of back so no worries there, but it does illustrate how unexpected things can happen.
I've tried a few OWB paddle type holsters, some OWB that the belt slips through, etc, and they're ok if you are dressing kind of heavy and plan to stay that way throughout the day, but they do tend to stick out a bit. I've done the arm pit thing as well and believe it's my least favorite to date
Personally I'm not extremely worried about printing some. I've stood right next to some of my city's finest in nothing more than a t-shirt and jeans, knowing I'm printing more than a bit, and never been called on it. People tend to see what they expect to see, and most people don't expect to see Suzy Q Public wearing a firearm, or John Q Public for that matter.