First:
You are correct. Your agreement is like a contract and cannot be unilaterally changed by one side without the explicit agreement of the other. The only unilateral action permitted is to walk away with no further obligations.
Next, I'm sure you used rather specific language in your negotiations about your financial obligations and expectations. When discussing "future needs" like tuition, supplies, and unplanned major expenses, I'm careful to say "If it's something you need, I will always try to help." The key words are "try" and "help." Try means I get to decide if I want to. Help means I may cover SOME of the cost, but not necessarily all. This leaves me options when asked for more. I can agree, partially agree, request additional time and activity in exchange, and I still have the option to walk away clean.
So you are spot on. Maybe this is a good time to switch to "mentor" mode and help her understand that financial obligations need to be analyzed and planned before making a commitment? But only of you see value in investing the required time.
Next:
I have now travelled to SE Asia three times, as well as Tijuana and various cities in South America several times, I recognize her song. This is what gig musicians call an Old Standard: Hook a "rich foreigner," fuck like he has the best dick on the planet, then start running the "emergency need" scripts. These scripts include such favorites as "Tuition", "Sick mother/father/baby/farm animal/uncle, etc.", and "Start a small business to feed my family in the province." The refrain for each of the songs is always: "Only you care, only you can save my ________________, I will love you forever."
Ah yes, the classic Foreigner Chump schtick. Gotta love the classics. Really the hardest part of these scams is getting the "mark" to believe his is the only dick she is sucking.
So hold the line and keep in mind that there are literally 1,000+ women celebrating their 18th birthday in your state today, and tomorrow and the next day.
Life is good
The Cat