This old chestnut again
Before we start, its important to note that in the same way some Hi-Fi manufacturers refer to "peak" watts in marketing, the true measurable and comparable value should always be RMS Watts, so Haldex refer to "Torque" in marketing terms as "applied torque against a surface", rather than the more generic definition of Torque.
The Haldex unit is a multi plate clutch device, capable of locking together the front (axle) drive and the rear (axle) drive. It is not a differential. Conceptually, it just joins the front half of the propshaft to the rear (propshaft).
1. When engaged, can the Haldex ever change the 50:50 drive (front:rear) to be biased more to the rear?
A: No. The Drive will always be, 100:0 (in the case of Haldex disengaged, FWD), or 50:50 (in the case of Haldex engaged, AWD).
2. Can the Haldex ever cause the rear wheels to turn faster than the front (assuming equal friction surfaces for all tyres)
A: No. Haldex can never make the car into a power-oversteering monster (assuming equal friction surfaces for all tyres)
3. Can the Haldex "send" more than 50% of the torque to the rear?
A: Yes (in Haldex marketing terms) eg: Front tyres are on a skating rink, rear tyres are on grippy tarmac.
In Haldex world, you can't "send" torque. Torque in Haldex speak is what is being transmitted from tyre to ground.
In the skating rink example above, the rear wheels will not be rotating faster than the front (ignoring the cross-axle diff), as the drive front:rear will be 50:50. But as the rears are gripping more than the fronts, then more torque is being transmitted by the rear wheels, than the front, and this will give you movement.
In other driving situations and scenarios, you can set up the car to get all 4 wheels spinning and drifting, and yes, through driving and momentum, you can hang the back of the car out (for a period of time) - but on a dry grippy road, assuming equal traction at all 4 corners, most of the torque will be transmitted by the front tyres before it even thinks of going to the rears.
Thats not to say that the car will always feel like a FWD car. Through careful setting up of the corner, you can utilise the fact that the rears are going to take a shorter path than the fronts (and turning slightly slower than the fronts), and you manually engage the Haldex to be ON, you can force the rears to turn at the same speed as the fronts, and you will "feel" the rears pushing.
So. If you go by the definition that you define torque by what is being transmitted between tyre and ground, then yes, with a Haldex Unit, more torque can be available at the rear tyres than the front, depending on the friction (mu) of the surface.
However, with the Haldex engaged, you can never have more than 50:50 drive (front:rear), and the rear wheels can never (in a straight line, equal mu surfaces) be made to turn faster than the front wheels.
The following video illustrates torque transmitted at the tyres, as seen by Haldex.