Personally, for a hill like that, I'd run the motor thru the gears, whether that's a BBSHD or some other existing middrive, or using a hubmotor as a middrive (see the hubmotor middrive media group thread for ideas).
That way you can gear down so the motor isnt' bogged down at low speed and wasting lots of power as lots of heat, which can kill them. (and wastes your battery). The lower you can gear the motor, the easier it is on it (just like trying to pedal up that hill, you're going to be in the lowest gear you can possibly get).
But you can gear up for higher speeds on the flats.
Since you don't need actual high speeds, you can gear the whole thing so that the motor has really low gears, and the highest gear is the highest speed you'd run at.
Your main limitations on this are the chainline from gear to gear with some of the existing middrives, but you can make up a "custom" front triple with the chainrings you need, even if you have to get (or make) adapters to fit them to the crank spider (or replace the crank with one that fits what you need), including using spacers on the rings to get them to line up with the rear sprockets correctly.
As far as limiting to legal power levels with a big DD hubmotor...there's no point to that kind of motor if you're going to limit it's power that much--it won't be able to do the work you need it to, especially on that hill.
A geared motor with a high enough gear ratio and a slow enough winding, or a middrive thru the gears with the right chainring/sprocket low-gear ratio, might be able to do it limited to lower power.
My guess is that to make the climb on motor power you're going to end up with an unlimited power button somewhere, but you can always experiment with it at the other power levels first.
LImiting power is easily done in the CA3.x, and you can make presets for various power levels/etc, or have a knob or switch for that. Teklektik's UUG (linked on the CA page at Grin) goes into a lot of detail on exactly how to setup various options. It'll also do torque-sensing BBs of a few kinds, and as noted in a previous post, there's info about adapters/etc for eccentric BBs.
Regarding loss of traction on the front wheel...I don't really have hills to worry about here, but we do have slick roads when it rains, especially at intersections where cars sit dripping oil, and that's usually where I have to make my turns. On CrazyBike2, which is 2WD, I found the hard way I have to back off or shut off the front motor approaching those turns, and not power it back on till I'm exiting the turn, or I could wipe out the front end, or simply lose steering entirely, etc. It's a lower bike than a typical uprright, so a sliding crash isn't a big deal by itself, but in traffic it would really suck.