missed segments on RTW
 

If I have a DONE4 and have ticketed it with say side trips in NA or Eu, and then mid trip by the time I get to those segments I decide not to travel them (eg. due to time constraints) can I just not travel them and keep going on the next segment I want to or will the relevant airline mark me as a no show and cancel the rest of my ticket?

Eg. - MEL-LAX-LHR-FCO-LHR-RIX-LHR-MEL but instead I just want to go MEL-LAX-LHR-MEL

You should ring the airlines concerned and cancel those flights. After cancelling you may want to call back and have them check that your remaining flights are still showing.

We had a few problems with LanChile not registering our cancellation phonecall. We showed up at the airport and were told "their system" showed that we had no flights. We explained that we had called them previously and cancelled some flights and they reinstated our bookings. Then on our next lot of flights we were told that as we hadn't cancelled our flights (the original cancelled ones) that all our flights had been cancelled. After explaining the whole saga again thay were reinstated.

Another reason to book all segments after the first as open.

Some airlines will insist on reissuing the ticket (and pay the reissue fee), others will pull the extra (skipped) segments with your next flown segment coupon (no reissue and no fee). If you need a reissue it will take some time (several hours typically). Some airlines will not let you board until the reissue is completed. It seems to be the luck of the draw which procedure you will get.