Talend is licensed as GPL... so 1) and 2) are illegal as there is a copyleft clause in GPL. Your code would have to be open source
Of the other ETL tools Kettle is LGPL, that allows you some more freedom. Most of the others are also GPL.
1. Am I allowed to leverage off and modify Talend source code to build my own data integration product? I intend to commercially license the product (at a fraction of the cost of the industry heavyweights) or even better sell it for a decent price to the organisation. I do not intend to expose the source code of my product for competitive reasons.
2. What commercial & legal implications (if any) are there of modifying part of the Talend front and back-end features and functionality and exposing the other parts unchanged, the way they are presented in Talend?
3. How long did it take and how many intelligent folks contributed to this interesting piece of software? This will help me estimate the effort if I need to build the custom ETL tool as per my original plan.