I'm not sure if the Call Waiting problem is actually a Call Waiting issue. The reason I say that is, if you're able to activate it on the handset then this means it's active on the network. Usually you'd get an error similar to the one for call forwarding.
It's possible the call forwarding is interfering with it, which I've not seen before, but if that's enabled then all busy calls would go to voicemail instead of showing as waiting.
This sounds like it could be related to the voicemail diverts which I've gone ahead and reset for your number. Please restart the handset now and check again for me.
For the diverts, you'll see forward when busy, unanswered and unreachable and these will need to have the VM Voicemail box number of +447458333222 (for the new purple SIM).