Hello Stephen.The company toyed with GP ratings, though I never sailed with them as it was considered a failure. One Chief I relieved said he could never get hold of any to work down below! All GP ratings I heard of were still deck ratings, none of them had been engineroom.
I've checked out Dillon since, and nothing in his history makes me change my mind Jim... Titanic was his first job down below - why he had changed from AB, who knows? Also he was in the engineroom probably because he hadn't a clue by that time of what the trimmer's job was - certainly much harder than an AB's. I still don't think he would know whether the engines were going ahead or astern as the two reciprocating engines rotated in different directions, and he would not have known that. He stayed AB for the rest of his time at sea, finally making Bosun it seems. Hardly anything there to show that he was any different from any other AB.
I sailed with British crews for the first few years I was at sea. The deck crowd were usually good and proficient at what they did. The engineroom crowd were more or less "Chambermaids with balls" as one Chief put it - all they did was bag down the plates and wipe oil off here and there plus dhobying the boilersuits - some ships were Brits (container trade), others Yemenis (on the company ore carriers). The only engineroom crews who knew anything about the engineering were Filipinos who had been in their own MN as engineers. I've sailed with Brtis, Cape Verde Islanders, Indians, PRC Chinese (the worst), HK Chinese, Yemenis (from Saudi Shields) and Filipinos. Most were good crew for what they did (apart from the PRC Chinese) and were no trouble - most would muck in and help with taking stuff apart and cleaning it if you showed them what to do. Dillon had only been in an engineroom for less than a week?
Back in the day, and right up to my time - before the advent of Engine Room Cadets, enclosed control rooms and even bridge controls - Marine Engineers below the rank of Second, were either time-served Fitters or Turners.
Engine Room ratings did not simply do as the ones you describe but were actively engaged with the Juniors carrying out maintenance on the deck and engine room auxiliaries. Even in my day, as a Deck Apprentice, I had to do the same... help lift and replace drive shafts clutches, change shell bearing etc.
Dillon, as you say was on his first trip as a Trimmer. However, he was unable to carry out his normal duties, so was seconded to the engine room.
As you know, when in the engine with the ship underway, if there were no side covers on the main engine frame, the big ends, cross heads bearings and shafts were visible for all to see.
Consequently, when the first engine order came, Dillon would see the the big ends turning the shaft on the main engine nearest to him, slow down then stop. There would be a "deafening silence" except for escaping steam and the shouts of the engineers as the changeover was taking place. Then the big ends would start to turn the shaft - but this time, in the opposite direction from which it had been turning continuously since they had left Queenstown. He would have needed to be as thick as the proverbial in the neck of a bottle not to have notice any difference.
You tell me he became a Bosun, that tells me he was a bit sharper than the average lad. I get the distinct impression from how he gave his evidence that he was a very precise individual who prided himself in his abilities.
The one thing he could not have known without seeing the ER telegraph, was the value of the astern order. As you also know, when bring a ship to a halt, you first ting down FULL astern then watch the stern wash and any tell tale over side discharges. Very often, especially when combined with a hard over turn, she ship slows much quicker that she would if kept running strait, Consequently the way comes off her much sooner and the astern movement is no longer needed. Think about it.