Hello chrisjgilmore!
I think to choose a knife for SD or as a backup for your P229 and pepperspray is a good idea. If you get cornered by a surprise attack you're lucky if you can draw anything at all and at contact distance, what it would probably be, it would be quite useless to go for your gun (remember the old 21 foot rule). A knife is the more dangerous and efficient weapon at contact distance anyway.
A symetric double edge is in my opinion more effective than a single edge because you can cut in both directions and it's the same efficient knife no matter from what direction you get a grip on it (weak hand draw for example). To be able to cut in both directions at the attackers arms, hands, throat, ...whatever is a clear advantage, but from my experience a full size combat dagger is quite useless for concealed carry.
I sometimes carry a slim compact dagger (fällkniven G1 Garm, Böker/Magnum Submariner) eighter on the Belt inside with TekLock or IWB. After handling my
large CS GFX push dagger (forget it) i ordered myself the large CS Safe Maker (still waiting).
You can carry a push dagger pretty concealed IWB. The grip wont poke you like with a normal dagger and the draw and punch are faster too. It also will fit your natural combat stile as mentioned before. I also agree that a push dagger is harder to disarm, BUT if somebody gets a grip on your hand, the push dagger is quite useless due to the angle of it's blade, while with a normal dagger you still can cut the attackers arm. I have to admit there are by sure techniques you simply can't do with a push dagger but that's of small concern here. It's simple and effective, so if you feel comfortable with it, get one. If you're more comfortable with a dagger, better get a compact one for concealed carry.
The fast draw, natural punch and conceiled carry advantages are also valid for the Ka-Bar TDI. It can't be hard to get a second edge on a large TDI if you want one. Though not symetric this knife feels good and natural in the hand. It is worth a closer look. I'll get one.
O.K. i'll stop bobbling and come to my recommentations:
Cold Steel Safe Maker (the large one)
Böker AF-Boot (Yep! they make a smaller version too)
Katz Avenger
SOG Pentagon (only if you feel comfortable with the Lack of a finger guard)
Ka-Bar TDI
SIG P239 (smaller than your P229, and a SIG like yours)
Glock 26 (i love it)
EDIT: Blackjack Blackmoor Dirk 4.5" (i forgot that Blackjack is up again)
Just try out what works best for you!
Of course a weapon is no substitute for training by itself, but you already know that. A Knife can't be a substitute for a gun in every situation eighter and you probably know that too. I'd suggest to take a look at subcompact guns too.
Please don't forget that there is a high possibility that you land on your back in hand to hand combat. I would not want to carry anything on my belt on the 6 o'clock position because it could cause serious injury to the spine.
EDIT: Of course you can mount anything in a kydex sheat with a TekLok horizontal on your belt if you like that.
Stay safe!
RogerRabbit