is this sentence grammatically correct ? [Tense]

Solution 1:

In case you are trying to apply here rules applicable for indirect/reported speech ("I always drink coffee", she said. She said that she always drank coffee.)... you don't have to.

...how excruciatingly unimplementable that decision is. (The decision is still to be implemented. But the implementation will inevitably fail.)

...how excruciatingly unimplementable that decision was. (The decision should have been implemented. But the implementation has failed expectedly.)

...how excruciatingly unimplementable that decision would be. (The decision has not been made at all. It remains theoretic.)