Can I use pure fruit juice/juice concentrate to sweeten dishes so as to avoid bad effect of sugar? Is the fructose, glucose in fruit better than sucrose

Not really... Sugar is sugar. That's why people who are avoiding sugar should not drink fruit juice. They are usually ok with eating small servings of fruit. Processed foods from the store that say "naturally sweetened with juice" and use grape juice (or another juice) are bad like products using sucrose. The best thing to do is to train oneself to enjoy unsweetened or only slightly sweetened foods and drinks.