Reapplying for a UK Standard Visitor visa after a refusal

How do I prove that I will return?

You don't. It's the result of an overall assessment of your circumstances, and trying to 'prove' too hard can backfire. The best way to demonstrate your intentions is to have a high quality application and a stable, independent life in India where your finances are inexorably tied to the Indian economy.

Can anyone suggest what should I do and what sort of documents to submit if I reapply?

In your various posts here you have already identified most of where the problems arose.

For your paragraph 1, about your income, they are worried about where about 1/2 of it isn't accounted for. Are you accumulating lots of cash every month? Otherwise show the movement of cash between your savings and current accounts.

For your paragraph 2, about what the trip is costing you personally, you wrote an amount that the ECO found wholly disproportionate to your circumstances. You should put more in your savings so that the amount you plan to spend is about 1/12th - 1/20th of your reserves. 1/12th - 1/20th is an approximate guess by the way, there are no minimum or maximum percentages driving their decision and each case is different. Some people probably get by spending a larger percentage of their reserves, but not you, so you need to increase your savings base.

Paragraph 3, about your not travelling very much, is just icing on the cake. They do that sometimes. You can simply include a statement explaining it, but it's not a big deal if the rest of your application is in order.

Paragraph 4, about your brother, he needs to submit his bank statements and a letter of attestation. He is not qualified to support you in the UK, so make sure his attestation is limited to purchasing air tickets before you leave India.

Paragraph 5 tells us that they are worried that you will be looking for work and possibly work illegally. You can offset this by assuring that your other evidence is really high quality and shows that your well-being is tied to the Indian economy. Given that you failed on V 4.2 and V 4.5 - 4.10 inclusive means they do not think you are a bona-fide applicant (that's a bad thing). Probably something turned up when one of the assistants was verifying your claims or your evidence or your evidence was so weak they could not verify it at all. There's no way for us to know (and we don't want to know); you might want to arrange a consultation with a member of the UK Law Society and pursue it further. Using the Law Society prevents your getting scammed by a con artist.

Is there any chance if I reapply?

I don't predict a favourable outcome if you apply again right away. You need a substantive change in circumstances and evidence that you are leading an independent life. If you apply again right away, they will refuse again and then matters will be worse.

Is it a good idea to buy return flight?

No, not at the application stage. If you try showing them tickets at the application stage, it's tantamount to admitting that you didn't read the guidance (a bad thing).

General Notes about New Delhi

The BHC New Delhi has a great visa section, arguably one of the best. Their turn-around time is 100% on target and they have 0% lost documents. They have a great review procedure and they rarely issue a visa to someone who should be refused (and rarely refuse a qualified applicant). Their refusal language is precise and accessible. When they refuse, you can be sure that the application had serious deficiencies that could not be overcome by phoning the applicant and asking for more stuff. Overall, the visa section is a professional and heads up operation.

I cannot say the same about 'visa specialists' operating in India.