Bad news regarding the landline. I must have heard incorrectly because shortly after we received an email from the PCCW representative to confirm our order. The price for the landline is $90/month, NOT $19/month. It's useful but not THAT useful.
The PCCW service will be installed on Monday 17th July. I will report back at that time to let you all know what the AC68U router is capable of. I don't expect it to be sufficient to provide WiFi to the upper floors so I will consider trying powerlines first.