(g) If any device is used that depends on the operation of any engine, and the landing distance would be increased when a landing is made with that engine inoperative, the landing distance must be determined with that engine inoperative unless the use of other compensating means will result in a landing distance not more than that with each engine operating.
I believe that the answer is in the above from post 4.
Reverse depends on the operation of that engine. Without that engine and it's reverse, landing distances will be increased.
How much reverse can you have on the operating engine with the other feathered? If anything it will be limited.
Thus reverse available from both engines is affected by the failure of any one engine.
So you can not use the reverse to claculate the required distances because it "depends on the operation of any engine".
I expect that your aircraft does not have something else to compensate for the loss of revese so unless the certification people have managed to get the figures with reverse approved for normal dry ops and thus in the flight manual, you are stuck with the figures as published.
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Just to correct one small point made earlier - the 1.42 factor does not give a 42% safety net.
A 1.42 factor means that you will stop within 70% of the runway leaving 30% of the runway available as a safety net.
Regards,
DFC