I built the system of reputale a Mexican startup that did online lending, so I have some insight:
1. This is a real problem in Mexico: people complain about high interest rates. However, for these kind of bullet loans , you see 30% of default in the FIRST loan. And back when we were starting on 2013-14 we had up to 70% of default due to fraud.
So, with around 30% 1st default and about 15% 2nd default, we created revenue models that considered that, CAC, cost of funds and opex to define the interest rate... that very high APR really was not making people rich overnight.
See, one problem in Mexico is that defaulting in a loan is not a crime, so theres legally NOTHING a lender can do to make you pay if you decide not to. The only repercussion is that you Credit Buro gets tainted, for 6 years. I've read of people that get a $100,000MXN loan and then just change phones, move apartments and wait for 6 years to clean their credit buro.
2. Default loans deffinitely are sold, for cents to the peso. But usually only the ones that are very old. Now, there are different types of collection agencies. Where I worked, we tried to he very reputable so we only chose the least shady ones. But in reality the only thing they could do is call you. Even calling your provided references is illegal. We actually stopped asking for references IIRC.
3. I dont know but dont doubt it. BUT, have a look at Fineract/MifosX: an open source loan management system. Its relatively easy to setup a lending operation using that. It's pretty comprehensive.
All in all, there are very shady app/online lending services in Mexico. But it is nothing exclusive of the internet. Way before this, you heard of lenders that would do "collections" by getting some tough guys from prison (yup, in some prisons they can get out for the day like nothing, for some cash) and knocking at the debtor's door with baseball bats, wrenches or any other hard weapon . If you didn't pay they broke your legs or worse.
That now it happens online is only the natural evolution of the shithole that is my country, unfortunately.