1. I usually begin zoning for medium density residential in the most centralized neighborhoods, which are closest to commercial/industrial zones, and have good health/education services in them. Generally, once your city is profitable and has a fairly established industrial/commercial zone, you can go ahead an rezone.
2. I've always expanded into many different lots from the beginning of the game. I've usually made one city R$ and industrial and the neighboring city R$$/$$$ and CS/CO
3. I keep my taxes for R$$$ high and only decrease them when I have established CO in my city. Even then, I don't let too much R$$$ develop, since it's quite unstable (as you mentioned).
4. R$ demand is almost always high. All commercial and industrial zones need R$ workers, so there's really no point that R$ will drop down. Instead, if you'd like your city to become wealthier over time, increase the taxes for R$ later on and watch as R$$ redevelops these buildings