There seems communication gap between you and gf! But not important here!
Yes deposit is 2 months and 1 month rent in advance! So totally you pay 3 month upfront, most of time first month when you check you only pay utility bills and when you end contract they deduct rent and utility bill from deposit , it can also be via a versa!
I will suggest you that this is normal.
This is Thailand! Once a foreigner always a foreigner
you know what all required documents are then what is you post here for? Accept it that it is case to case basis! You came here for 15 days so there is no scope of extension! Except the agent that you mention in post - but have to willing to pay! Again indian passport soβ¦! Keep smiling and keep visiting