Phuket Sandbox: will the hotels allow my Thai girlfriend to come stay in the hotel with me as a domestic traveler from Chonburi if she only received one AstraZeneca shot so far?

One of the two hotels that I booked said this:

My Question #3) What process must I follow to obtain the Certificate of Entry, and what do I need from your hotel to complete my application for the COE? (Do I need to follow some process or obtain something for my COE application from both your hotel and the other hotel that I booked for Sep. 1-12?)

- Please do check and follow steps for COE registration at https://coethailand.mfa.go.th/ .

- After you get COE pre-approval, you can book and fully paid for accommodation to both hotel. And then each hotel will issue SHABA certificate per person for COE application and for you to purchase RT PCR test."

This raises two questions in my mind: 

1) I don't need to mention anything about my girlfriend's name or information in the COE application, right? Even though she will be staying with me in the hotels if we're lucky enough for domestic travel to be reinstated by September?

2) The second hotel that I booked for 3 nights was a very expensive romantic package. I can cancel for a full refund until September 7th. So I wanted to keep the reservation in case my girlfriend's ability to join me is reinstated before then. If it's clear during the first week of September that she still won't be able to join me, then I want to cancel the 3 night booking at the second hotel and just extend the booking by three nights at the first cheaper hotel (because there's no point in spending so much to be at the second hotel by myself).

If I've already been admitted to Thailand with the COE etc. and obtained SHABA certificates from both hotels, will I run into any problems with the government (or in keeping the PCR test appointments, etc.) if after arriving in Phuket I cancel the booking at the second hotel and extend my stay at the first hotel, in order to cover the whole sandbox period more cheaply while trapped on Phuket Island alone before leaving for Bangkok?