You're asking the right questions. I think the current view is that most patients have lost at least 90% of their beta cells by the time they develop symptoms and are diagnosed with type I diabetes. The remaining 10% of islets continue to function (secrete insulin) but are gradually lost over the following 1-2 years.
Originally, islets were not thought to regrow, but they have now found islet progenitor cells even in patients with long standing diabetes. The thinking is that any new islets (if any) are being destroyed as fast as they are developing.
As I mentioned previously, it is possible to identify high risk family members (with islet antibodies) many years before they develop diabetes. Treg therapy could potentially prevent or at least slow the time to diagnosis. They may also have abnormal intravenous glucose tolerance tests as they get closer to diagnosis (see below). The problem is that most patients (?90%) are the first in their family with the disease.
Islet transplants, however, could really benefit from a Treg therapy, to both prevent transplant rejection and a recurrence of the autoimmune process..