Eliza,

welcome to Flyertalk!

1. Never a problem to buy other ticket - I usually have 2 or 3 RTWs open at the same time and buy OWs or RTs in between. As long as you follow the rules for each of the tickets (use coupons in sequence, don't leave cupons out - except on the end) you will be fine.

2. It really depends on the route. Some flights sell out quickly in the RTW classes, some are available even on the same day. So it really depends. Since date changes are free i would recommend to issue the ticket open, then a few month book a flight and then change the reservation as often as you want.

3. They depend on the airports and number of stops. A transit is usually cheaper than a stop, also different airlines have different fuel surcharges. I have a great travel agent who does all my tickets and he is usually within 10 % when guessing the amount of taxes. On my last RTW taxes have been around 800 US$, but yours can be very different.

4. If you plan to do multiple changes, change the routing etc i would recommend to use an experienced travel agent. Emailing the agent what you want to have changed is far easier than spending hours with the airlines on the phone when travelling. Since i typically change the routing on an RTW at least once and have numerous date changes i believe it is well worth the fees the agent charges. If you don't plan on changing much around booking online or direct with an airline (AA seems to be good) is cheaper.

Btw, you are wasting HKG-JNB. Lots of miles.