Recently I have been assigned to work on a major requirement that falls between a change request and an improvement. The previous implementation was done (badly) by a senior developer that left the company and did so without leaving a trace of documentation.

Here were my initial steps to approach this problem:

1. Considering that the release date was fast approaching and there was no time for slip-ups, I initially asked if the requirement was a "must have". Since the requirement helped the product significantly in terms of usability, the answer was "If possible, yes".
2. Knowing the wide-spread use and affects of this requirement, had it come to a point where the requirement could not be finished prior to release, I asked if it would be a viable option to thrash the current state and revert back to the state prior to the ex-senior implementation. The answer was "Most likely: no".
3. Understanding that the requirement was coming from the higher management, and due to the complexity of it, I asked all usability test cases to be written prior to the implementation (by QA) and given to me, to aid me in the comprehension of this task. This was a big no-no for the folks at the management as they failed to understand this approach. Knowing that I had to insist on my request and the responsibility of this requirement, I insisted and have fallen out of favor with some of the folks, leaving me in a state of "baffledness".

Basically, I was trying a test-driven approach to a high-risk, high-complexity and must-have requirement and trying to be safe rather than sorry. Is this approach wrong or have I approached it incorrectly?

P.S.: The change request/improvement was cancelled and the implementation was reverted back to the prior state due to the complexity of the problem and lack of time. This only happened after a 2 hour long meeting with other seniors in order to convince the aforementioned folks.