The biggest mass on the bike is the rider and if you start hanging heavy stuff on the bike a long way from the rider then the farther away from the rider and the heavier it is the greater the change to the way the bike handles. If you can get that mass very close to the rider then it's almost like the rider just being a bit heavier, at least when in the saddle.
Yes, but you will probably really notice it when out of the saddle if you tend to rock the bike side to side. Some riders do that when they climb and some don't so it may or may not matter much depending on that and obviously how much weight we are talking about is a big factor as well.
Putting heavy stuff low down makes the bike fee sluggish, but it's stable and that was always the touring norm because weights and volume could be quite large and any practical solution meant that bike handling had to be sacrificed.
Yeah, I agree. Also if the weight is down low it probably won't be noticed as much when rocking. Again, this may or may not matter much depending on whether the rider. likes to get out of the saddle and rock the bike side to side.