Does treatment with arb's specifically olmesartan 20mg reduces proteinurea to below 30mg even if secondary hypertension is still there in arteries?

Proteinuria: Olmesartan (o) reduces both proteinuria (p) and blood pressure and it does so in a dose related response. That means the more of it you give, 40 mg instead of 20 mg, the greater the reduction of p. Speak to your doctor about the advisability of increasing the dose of o in your case to reduce your p. I am not sure of what you mean by "even if secondary hypertension is still there in the arteries".

Probably: Hypertension caused by unilateral renal artery disease is best treated with an arb or acei. It should help with proteinuria also. If severe proteinuria is present tho, you should be looking for another source of proteinuria. Talk to your physician about this possibility.

Yes: ARBs and ACEi class of drugs reduce proteinuria regardless of the cause of the hypertension (they restructure the anatomy in the glomeruli which are the building blocks of your kidneys) and make the filter in your kidneys tighter (so to speak) to prevent leakage of protein in the urine.