1. Currently unless you have a COE then you can enter.
2. Quarantine is solitary confinement. Means no visits for 2 weeks.
3. Maybe if they confirm the Phuket or whatever place tourist plan, your Gf can visit after your initial 2 weeks of quarantine.
4. I suggest you wait till probably october/November then ask more about this, before more triggered people comments because you are planning for December, there's not enough information at this point of time.