In 1) I assume you are dealing with dry metaled roads here. If the conditions are slippy, surely too sharp an application of the front brake will result in a front wheel 'skid' (not pleasant). MTB maniacs do this all the time but stay mostly just on the right side of the skid/no skid loss of control threshold.
In 2) (assume 'right' = 'rear') the rear wheel only lifts if you mostly apply braking effect to the front, and don't sit back when applying hard rear brake. Anyone can get their rear wheel to stay on the ground and skid, even in the grippiest of conditions (not good for the tyre, mind).
Have I failed to understand?
The topic is road biking and what I said thus applies to road biking. I'm not familiar with the term metaled and my cannot skid" term applies to dry asphalt. Gravel, marbles, ice and sleet are exceptions to this rule. When a tyre skids on a MTB on dirt it doesn't test the coefficient of friction of the rubber but rather the shear strength of the surface you are skidding on. In these cases any rubber will perform exactly the same (grippyness is irrelevant) unless the knobblies can imprint deep enough to either increase the shear strength of the substrate or, bypass the loose gravel and activate the weak atomic forces that gives a tyre traction on asphalt. Imagine a bike riding on a bed of bearing balls. If you brake, the balls roll and the rubber itself is irrelevant. The only factor a tyre can contribute in such a skid is the shape and depth of its knobblies. The deeper they dig in the more shear strength they can muster from the substrate. On a road bike where the tyre does not imprint on the substrate, it is purely down to the rubber's Van der Waal's forces with the road or, grippyness in layman's terms.
2) Most people only test their brakes in an emergency and in such cases there's usually not enough time or conditioned reflex training to move backwards and get weight over the rear wheel to give it a bit more traction. Thus my example applies to that situation but I think you have a valid point and if I may paraphrase you "doesn't hanging over the back and skidding the rear wheel test the tyre's adhesion?" The answer is yes and no. Hanging over the back this won't test the tyre's adhesion in a controlled way and with a good or a bad tyre you will never get a corresponding good and bad reading. The difference in adhesion is so small between good and bad grip tyres when there is minimal weight on the rear wheel, that it is completely dwarfed by the huge errors in measurement you create at the body's position and applied brake force. You can induce sliding by the force with which you pull the lever or by adjusting your position.
How is the layman gonna control that and come up with a "reading" and make a decision on a tyre's grip and thus safety.
Smoking Joe is right about gravity but the rear wheel still receives reduced downforce even if you only pull the rear brake.
Marquis' rear wheel drifts because the tyre could not muster up enough shear strength from the dirt he was cornering on. By changing tyres he changed tread pattern and knobbly effectiveness and that improved traction. However, the grip under those conditions didn't improve because the tyre had a better coefficient of friction. Remember, whenever the tyre makes an imprint on the road, it is down to the knobblies digging in. If the tyre doesn't make an imprint, grip is due to adhesion.
