The Midland borrowed M&GN tank loco(s) for a while, operating the Wirksworth branch IIRC - not sure if they ever got to Derby on a regular basis. I think the furthest west that proper M&GN trains got on a regular basis was Nottingham.
I think some S&DJR locos were built at Derby, so would be seen there - if only briefly.
The obsession with getting things absolutely right is very unhealthy IMHO. Much safer to model a line for which you can become the only real expert. The Hull & Barnsley is probably getting close - Lancashire Derbyshire & East Coast or some other obscure line that almost no one has even heard of might be even safer. Or some obscure Siberian line in the 1950s when anyone taking photos or information might not live to tell the tale, even if they could ever get that close?
2 different bridge designs might be interesting and appropriate. The siding could be an original, rickety wooden or iron bridge and the main line upgraded at some point to steel girders.
Her's an interesting bit of pointwork from Sweden. I thought single blade turnouts went out with the Ark, but this one was in existence between the wars. No exact date though.