Can Loosening of proximal contact after final setting of prosthesis be predicted or reduced ?

Unpredictable. But there are some ways to reduce loosening.

1.Occlusal adjustment - formation of stable occlusion.(preventing premature contact)

2.Regular re-tightening.

3.Increase contact surface with adjacent tooth to make stable contact.

Does it have any clinical significance to cover defect that 1 or 2 thread is exposed with GBR or any other ?

It has significance on preventing bone resorption.

In case of mild thread exposure, grafting small amount of autologous bone is effective.

When there is severe exposure, we recommend grafting autologous bone + OSTEON™ III or autologous bone + OSTEON™ 3 collagen