If the LAD is 100% occluded because the person is actively having a STEMI (and the LAD appears to be the culprit), then you try to open it. If the vessel is chronically occluded, then whether or not you try to open it becomes a controversial issue that depends on many factors, including whether the person is having angina and there is no other good reason for it, whether you think there is any viable myocardium to save out there, whether there are good collaterals, etc. Opening chronic occlusions has become more commonplace and technically feasible in the modern day, but I personally think it is done far more often than can be justified.
As far as what can be opened: these days if you want to use the latest toys, you are well-trained in more exotic techniques such as retrograde approaches through collaterals, you're very patient, and you and the patient are both willing to possibly give it several attempts and get yourself some radiation, then there are probably very few occlusions that literally cannot be opened in the cath lab. But again, you have to ask yourself why you're doing it.
As far as the lowest % that can be stented... obviously you can stent any minimal narrowing that you want to. If the lesion visually appears less than 70%, this may be malpractice. If it visually appears less than 50%, it is almost certainly malpractice unless you have some other modality (i.e. FFR) that tells you that your eyes are fooling you. The practice of stenting moderate (or non-existent) lesions has landed cardiologists in prison. (See: United States v. McLean)
