I will be only writing what I have learned so far and if in any way it is wrong or misunderstood it is not the fault of my instructors but my fault because of my poor understanding of Aikido.
1-Now when it comes to attacking while Uke is attacking Uke must commit to the attack if Uke did not commit to the attack it will be of know value and Nage will not be able to learn the technique or the principles behind it. This will be hard for some beginners since at the start of Aikido they will feel that it is unrealistic to do that. They do not understand that its a way to learn more realistic training and situations will come later. That should be discussed between them and their Sensei.
2-If Uke suddenly decides to change the attack or to shift his weight to somewhere other than the attack Nage can either adjust to be able to do the technique or switch to another technique since the attack or the commitment of the attack has changed. Aikido flows and adapts to situations.
3-If Uke is experienced Uke will be more compliant with beginners since they need to learn the motion behind the technique to learn the principles but after a some time(maybe a few months) Uke will be putting up more resistance to make the attack more realistic(some practitioners ask for more resistance from their Uke) and to make sure Nage has good grasp of the principles. That is one of the ways to improve.
4-GO FOR NE-WAZA ITS FUN!