(1) Friction is measured by taking the down weight and subtracting the up weight, then deviding this number by two. For instance: U/W=52 grams. D/W=26 grams 52-26=26\2=13 grams of friction.
(2) Two pianos with the same down weight can feel different for several reasons. The hardness or softness of the hammers can give the impression of a different feel because you would have to strike the mellow piano harder to get the same volume. Also the friction in the actions could be different or the inertia could be different or the action geometry could be different or probably several other factors as well.
(3) Action geometry is the total ratio of the three levers in an action. For instance, if the front of the key is depressed 1mm and the hammer raises 5mm, that would e a 5 to 1 ratio. This can change the feel because as you change the ratios, the key dip or the blow distance of the hammer will change. also, the weight changes as well.
(4) This is how inertia was explained to me. If you had 2 teeder todders, whitch work just like piano keys by the way, and on one you had a 50LB child on one side and a 60LB child on the other side you would have a 10LB difference. On the oter teeder todder you have a 300LB man and a 310Lb man. There is still a 10Lb difference but you would be able to push the one with the small kids up and down a lot faster than the one with the big men because there is a lot more inertia on the second one. A piano key has lead wights installed in it to balance the weight of the hammers and other parts. Changing the amount of weight in the key changes the inertia.
(5)Down weight is the amount of weight it takes at the front of the key to overcome the friction and weight of the other action parts to lift the hammer to the point of letoff.