Not very good. The rain nowadays are more acidic than neutral, and if you swim in a pool of rainwater too much, it is bad for your skin and hair. And if you swallow it, the effects might be minor but slowly, you'll get sicker and catch colds.
impluvium is the answer
The rainwater will be treated by the chlorine in the pool and you may have to adjust the chlorine levels. See the related link for more information.
yes
Filter it then swim in it.
Yes
Yes, there are some companies who can do work such as this.
possible hydrostatic pressure sandy soil and enough rain water pool could lift up like a bucket in a bathtub full of water.
The more rainwater that is added means less salt water this results in diluting the pool. It also works vice versa!
Yes, pool water is bad for chipmunks because of the chlorine in it.
It will smoke really bad because the piston is getting to much oil and and it will run high..
Chances are you have a bad head gasket if that is happening. Get it repaired ASAP.