I'm coming in late to this discussion, but I suspect there may be confusion that might never be fully resolved.
To ring down a speed change, say, to stop or full astern, the bridge watch would move the engine order telegraph for the particular engines. A bell would ring in the bridge telegraphs. Then, simultaneously, the telegraph in the engine room would ring and point to the newly ordered speed. The engine room would move the acknowledgment pointer to acknowledge the order, and its bell would ring. The bridge telegraph's acknowledgement pointer would move to the ordered speed, and that bell would ring again.
So, in my mind's eye, I visualize some degree of pandemonium on the bridge. Multiple telegraphs are ringing repeatedly, various voice orders being given and possibly countermanded, the telegraphs being operated, and people running around and shouting.
Meanwhile, there would be major turmoil in the engine room. Bells ringing and unexpected telegraph orders being received. Changing the reciprocating engines' speeds would take time - and even more time to go from ahead to astern, with multiple valves to operate, requiring several men to race to their positions and execute the changeover that they may have only done a few times, and maybe never under emergency conditions. The ship had been operating several days with hardly any bells rung down to the engine room, so the engineering watch-standers are there in semi-daze, when all hell breaks loose.
I would doubt that any survivors could have really said exactly the order of events, and what the engine rpms actually did (not just ordered). And even if all had survived, there would be conflicting testimony.
In my ancient navy experience standing engineering watches, we had plenty of watch-standers to respond to emergency bells. I remember standing watch in a fireroom (boiler room), and an officer asked me what I would do if there was an astern bell during routine steaming ahead. The answer, was, Sir, cut in all the oil burners (the astern turbine was less efficient, and you could always cut back the burners if necessary to maintain steam pressure). Of course, that would not have been the correct answer on Titantic, but you get my drift.