You need to change your strategy, to get access to the right people. It sounds like you are approaching this head on. This is China, it doesn't work like that.
Someone from your uni should be helping you build these contacts. Unless of course someone is blocking you. Either way you should contact your American sponsors, who should be able to oil the wheels at least.
Ask people in your dept. at Yunnan Uni for help. If anyone says 'impossible' that just means they have not worked out how to do it. However, if someone says 'that is difficult', it means that there is a reason it can't happen. Although they will not be at liberty to tell you.

All of the schools involved in these programmes have a Chinese partner school. The protocol is that the Chinese public school is in the lead position in the relationship. The Principal of the public school should also be contacted. Not only is this courtesy, but they are also a part of the jigsaw and may be able to provide another point of view for your research. They do not need to be your gatekeeper however.
There are also the schools owners (international side). These should be contacted. Without this you will only be able to access foreign teachers who are willing to give you their time. Local staff will be unwilling to talk. Either because they don't have permission or they don't have to. If you take a top down formal approach, this can get you a much better result in China (hierarchical and context specific culture). That way everyone will be obliged to talk to you, if you can get the owners to participate. You may also be able to access some data, rather than just anecdotal evidence.
You also need to bear in mind that foreign teachers are more likely to be critical of local management. Consider the situation back home where staff are also very likely to be critical of management. Sometimes criticism comes because not all of the facts are known, despite more transparent management style. Then compare China, where there is both the language barrier and more opaque management.

Additionally, consider that local staff are unlikely to critical of anything that may reflect badly upon superiors and/or peers. Even if they feel strongly that there is a need for change

Rote memorization is really important for some subjects, learning language included. It has a bad rap, but is a valid learning skill, often neglected. If you go out and talk to most of the population, anywhere in the world, you won't hear 'good sentences' all of the time.
The real success is being able to communicate with others. Find yourself some local friends who don't speak English. Chat to old people in your neighbourhood. Shop at the local wet market, and ask about the produce when you do so. Not just 'how much?', but what is it called, where is it from, how to cook it (even if you know) etc.

The ice is thin and the wu mao are not so far away.

I would be very careful collecting this data for a number of reasons.

Firstly, I have seen students who fail because they don't work, don't attend, or do not have some other study skills. Others are just running away from failure in the Chinese system, i.e. they were destined to fail anyway. These students and their parents will often refuse to take responsibility and blame the school or the teachers or both.

Secondly, universities in many countries already complain that their home grown students are ill-prepared for university study. Common Core (US) and the National Curriculum (UK) are often blamed. Not the fault of the school or teachers.

Thirdly, some parents/students will ignore the guidance of the school when choosing their university/college or course, and so are may be setting themselves up for failure. Include choosing schools they cannot get into, or courses they are not prepared for. Some universities will accept anyone who can pay the fees, for any course.

Fourthly, the drop out rates from colleges are very high for all students. This is a fault with the system, not individual schools or their teaching.

Biased though it might be. Asking those students who have since graduated from university overseas how well their school prepared them, and what they think might have been useful in preparing them better could give more useful information. But you must also bear in mind, that you are asking questions from non-educators who are often uninformed about learning practices.

Perhaps the best people to ask would be the overseas universities. They see the gaps students have that really need to be filled. Again, not all of these gaps can be filled by preparation at high school level.