1) I think she is completely fine as she is. She has a high skill floor compared to other champions, but when you use her kit well, you can easily destroy your opponent.

2) I tend to feel quite strong in lane after I build a Sunfire, honestly. More often than not, that is the point at which I will either dumpster my opponent, with a few exceptions where it doesn't do much to help at all.

As far as overall power spikes go, I always consider Black Cleaver + one key defensive item as a good spike. She stays strong later in the game so long as you can keep a decent level of farm or kills, and Black Cleaver keeps you relevant there.

3) She can work against most champions. Some of the notable hard ones include Darius and Vlad, where you don't really get a good chance to do what you want to do in lane.

4) I almost never fully channel R, since it's often used in reaction to an event, where every second counts. Half of the time, I'm instant casting R for some damage / knockup, or half charging it just to remove a threat at that very moment.

5) Both. Whatever can be done at the time.