On the second point (the need to maintain light acceleration) I'd always thought this was more to counteract engine braking - if there is engine braking, that is more demand on the rear tyre, kind of like applying the brake - which takes up a bit of the "available" friction that could be used for cornering.
I can only really explain it intuitively, in that when you pull on the rear brake or engine brake, the angular forces of the rotation of the wheel are transferred into making your bike straighten up vertically. I don't know if that actually happens, but that's how it feels.
Part of it is also similar to the reasons you trail brake into corners, in that you're managing the weight distribution over your tires. You don't want to be erratically changing how much weight is on the front tires, otherwise you'll find you suddenly have a lot less front grip.
I can't speak to motorcycles, but when driving a car at the track there's "maintenance throttle" which is partly to counter the slowing you get from the tires scrubbing, and partly to balance 'weight'; most cars are far heavier up front than in back, so a bit of throttle puts a bit more weight on the back of the car, which helps keep the front outside tire from getting overloaded but also puts some weight on the back tires for more grip. It also offers the option to induce some oversteer via throttle lift, if necessary.
That’s right, in a car as you go into a corner you first steer a little bit, the car then “takes a set” - i have no idea where that terminology comes from but it just means the car moves to a steady state of loading up the outside suspension. Once you feel the car has taken a set, you can crank on a lot more steering. I’m describing it as two phases but the best drivers smooth both of these phases into one flowing motion - slowly steer at first then steer faster. Doing it in this two phase approach allows for far higher lateral g to be built in the turn than if you just smoothly turn at the same rate. You extract more grip because you build weight over the tire before asking it to grip and grip is parlty a function of weight on the tyre.
In most car drivetrain configurations you’d then use a balanced throttle through the turn.
You can use a balanced throttle on a bike too - many people do - but all race bikes and most advanced riders even in non race scenarios will use a slight acceleration not a balanced throttle because where in the car you want to settle the suspension with a heavy loading equally on your two outside wheels (e.g. in an FR layout car) on a bike you want to have around 65% weight on the rear.
There’s a similar effect with the Porsche 911 - it’s an RR layout with 12 inch wide tyres at the rear and a slight acceleration is optimimum for it.
I would say it's more to keep some weight on the rear wheel and prevent a low-side crash. When the bike is slowing down the weight shifts to the front, whether through friction braking or engine braking. If you are already at the correct speed, lightly accelerating helps bring you through the corner with more even weight on the wheels. Incidentally, race car drivers do the same thing, except when drifting, but you almost never see intentional drifting in road racing.
I always thought that it's simply because as you turn left/right, the bike will fall to the left/right, and the acceleration delivers an outward force that levers the bike up against the friction of its tires (and disappears once the bike is vertical) i.e. the same thing that keeps bikes vertical and straight by themselves as long as they're getting power. That being said, I'm a rider not a thinker.
Counteracting engine braking happens already at maintaining speed. Maintaining acceleration is maintaining acceleration, with the obvious effect of moving more weight on the back wheel and shortening the bike.
I think its for overcoming the braking that occurs when you turn because of the normal force. Even if you hold down the clutch the bike will brake more that if you run in a straight line.
