The term cure is actually pretty clear, but you are right it is often misused. To be cured means that the problem is completely gone, not just in remission. Too often people are considered cured of cancer just because the cancer disappears for more than 5 years. This does not meant hey are cured though. Many cancer viruses, and even some cancers, are very slow to cause cancer or grow. Breast cancer for example can take up to 10 years to reach a detectable size. And the SV40 virus can take 25 years or more to cause cancer. So in some cases people are falsely claimed to be cured.
On the other hand it is possible for someone to actually be cured. And it is possible for a condition to still come back after they have been cured. If someone had the flu and they took andrographis to destroy the influenza virus they can be cured of the flu. And they can catch the flu again later from another strain. Same with diabetes. Early type 2 diabetes can be cured by restoring the insulin receptors. But as you pointed out if they go back to the same habits that led to the development of diabetes in the first place then they can develop it again even though they were once cured of it.
The tricky part is differentiating between cure and remission. If someone follows a particular protocol they may feel better from that protocol. For example, the cancer patient may feel better from the tumor reduction after chemotherapy. Is this a cure? No. Do they feel better? Yes. Is the therapy risky? Yes. Can they develop long term side effects from this therapy that made them feel better? Yes. So just because a therapy makes someone feel better this does not make it a cure. So this is where people need to make the distinction. And this is why studies are needed to prove that there is a basis for a therapy. I am not talking about expensive double blind studies. The man that taught me about Quantum Touch, a form of energy work, wanted to prove the existence of the energy because it is a hard concept for most people to believe. So he paid a university to run some tests proving changes in substances exposed to the energy. The tests confirmed the changes. This may never be written up in a medical journal, but it is still a form of solid evidence that something exists. Those who come up with excuses against studies or providing other forms of real evidence are those who don't have any real evidence and are afraid of studies because they already know what they will reveal.