1) What Bingo said.
SW Wiki purports to explain:
A misconception exists that rather than being indicators, midi-chlorians actually are the Force or create it; there is no canonical basis for this belief.
Firstly, if that has ever been much of a misconception, it's not much worse than the truth.
Also, it makes them sound more like parasites than symbiotes. They're simply indicators, gathering in a Force-strong individual...
The rationale for midichlorians undermines itself as the Wiki also says:
When present in sufficient numbers, they could allow their symbiont to detect the pervasive energy field known as the Force.
So...they are indicators of strength in the Force...but without midichlorians...the person may not (can't?) be able to detect the Force. So we're still left with the question of what makes a person strong in the Force that causes Midichlorians to gather...and if the person is already strong in the Force, why do we need Midichlorians? Like Bingo says, "unnecessary badly thought out piece of pseudo-science." They add nothing but were supposed to push along the symbiotic narrative of TPM (which I thought fell flat on all levels).
On this background, the answers to your questions are...whatever reasons Lucasfilm wants to give you because it really isn't coherent.
2) What Bingo said: Anakin's a git. There were the bits about him not appreciating democracy, of wanting the (absolute) power to change things, but it was so offhand and poorly integrated that I agree it didn't seem like a motivator.
3) What Bingo said: bad pun.
I don't want you to think we are just PT haters - though a fair share of us are - but we've gone through the necessary stages of grief and decided that as much as we love Star Wars, there are some things we just can't abide. So ultimately everything that Bingo and Frink said.
