Talk of two passing loops, a double track section, five or six CCTV crossings (two different pages on their temporary website give two different figures, and Quail shows at least seven crossings on the line in any case!) and comprehensive signalling (and signalbox) seems rather grand for, initially at least, a proposed hourly service.
I don't know how the branch is currently signalled (presumably the interlocking still exists), but the most basic form of One Train Working would happily allow a train to pootle up there from Derby every hour. Providing something akin to the Matlock branch, with full token working remote from the controlling signalbox, would allow trains to be 'shut inside' at the Denby end and any intermediate sidings. In both cases, I'm sure that it would be possible to operate the branch fully from Derby PSB (as, for the most part, little more would be required than signalling the train onto the branch, releasing the token to the Driver, waiting for the token to be restored and signalling it off the branch!).
Level crossings - seven or eight CCTV crossings is an awfully large number both for installation, maintenance and supervision - and may exceed the number that can be operated by one Signalman under some agreement that exists? For a low-speed hourly service, crossing mainly relatively quiet roads, locally monitored crossings (ABCL or similar) should be sufficient. If that's not suitable for any of the crossings (the A609?), then some sort of trainman-operated crossing with barriers wouldn't put too much into the timings.
The lack of a connection from the main lines at Little Eaton Jn is something that the website fails to mention - I'm sure that the interlocking remains for these connections (indeed, I'm told that the signal on the Down Main in rear of Little Eaton Jn still carries the junction indicator, albeit bagged over), but there's still a fair bit of work and cost involved in installing two crossovers on a busy main line!
So yes, it does unfortunately appear to me as though the group behind the efforts (who have a strangely varied portfolio!) don't understand the cost of the works that they propose, and I do wonder how they hope to attract funding for a proposal that is full of features unnecessary for the service that it will initially deliver. The increasingly successful Ecclesbourne Valley Railway started with a similar setup, and it's taken many, many years for them to get to the point of having a service connecting with the main line, and I suspect it'll be a little while yet before there's any movement on through running south from Duffield.