USA B2 Visa for Thai GF and Parents

Hi there, my girlfriend along with her parents are having their first B2 tourist visa interview this coming Wednesday at the US embassy in Bangkok.

Before I give background info, I understand the chances are probably low.

I know there are no guarantees the internet can give for this type of question. But anyway...

Background: 

Myself: 27, US Citizen but also a Costa Rican citizen. I've lived in Costa Rica since 2011 with my maternal family, enrolled in University, basically this is my home. I don't have any intent on ever living in the USA. I only visit the US 2-4 weeks a year to see my paternal family. I go to Thailand once a year for a month to visit my GF. Been doing this the last 6 years. My grandparents and I are inviting her and her parents to visit us for 2 weeks during my vacation period. They (GF & family) were able to get time off, so they will be applying for B2 visas. So in this case, fiancee / spouse visas are not for us. We aren't looking to be residents of the US.  

GF: Thai, 28yrs old, College degree, Full time employee since 2014, lives with parents (only child), never traveled abroad, income is 20,000 THB a month + overtime. Speaks Thai, Eng 

GF Father: Thai, late 50s, Full time employee with the Government, never traveled abroad, salary 50,000 THB a month + overtime. Speaks Thai

GF Mother: Thai, late 50's, previously employed, never traveled abroad, Home Maker for the last 26 years, Speaks Thai, cares for her aging mother and nephews / nieces part time.

They fully own the house / land they live on and they own 2 cars ( definitely middle class). They have all the basic documents such as bank statements, employer letters authorizing them time off, land ownership documents, etc. No criminal record, no previous attempt for visas etc. They will fund their visit entirely themselves. They will be staying in my grandparents home. 

I know the CO won't care too much about my situation since it's their application. If they end up asking about me in depth to determine if I am truly a tie to the US, I also provided my GF with scans of my passport stamps showing how I've always returned to costa rica since 2011, Costa Rica national ID, school ID and school documents showing I'm actively enrolled. Also my grandparents provided a letter of invitation with signatures and photocopy of their IDs to confirm address and purpose of invitation ...

Questions/ Concerns: 

1. What sort of questions can my GF expect? Would it be of any benefit to speak English with the CO? 

2. How would you recommend she phrase the answer to "What is the purpose of your visit"? We don't want to lie about me, since visiting me and for them to get to meet my family for the first time is basically the reason. To do touristy things together too. 

3. Anyone with a similar situation that was approved/denied? Would love to know your experience. 

Thanks