Every designer can empathise with alot of the problems you're having.
Implementing the designs properly
"Developers create something that isn't quite how it was imagined by
the team during our brainstorming session. Designers become frustrated
when last-minute revisions muddy up their spotless layouts."
Here are some activities to help these problems which I've always felt were essential in the development phase. Personally, I do find that a non-trivial amount of my time is spent ensuring the design is implemented well...
As a developer picks up a new feature to implement, and before they start working, schedule a brief walkthrough and discussion of the designs they are about to implement.
This allows you to explain parts of the design that may have intricacies which could be missed in the design docs. This is great because a developer gets an opportunity to ask questions about the design that they might simply make an assumption about otherwise.
Some may refer to this as a 'handover'.
Midway through development of a feature (or at another appropriate milestone depending on the size of the work), schedule a time for the developer to run through their work with you. This isn't so formal, it's just a check-in. This will let you spot any problems or misunderstandings whilst the work is being done.
Make yourself available
This is really easy if you sit with/near the developers. Questions come up all the time. What I've found is that sometimes it's easier for others to make an assumption rather than to ask, particularly if it relates to a lesser prioritized part of the designs like the visuals (as opposed to functional requirements). If they can turn around and quickly ask you to clarify something, then of course they will. If they need to pick up the phone or write an email, the likelihood decreases.
If you don't sit directly with them, pop over occasionally and make yourself available, even if it's just to say hey. If you're remote, then just keep your communications up with them in whatever form you can.
What to do during development
"I find that the role of UXers quickly becomes idle."
Of course prior to development, and whilst you're in your early design phase, you would have done your user research including testing of some design iterations, and (as you've said) some hi-fi prototypes, with users.
But now as things start coming together, it's a good idea to go out and test the tasks a user is able to complete with the 'in-development' software.
You might ask why this is needed, and truthfully, there are many companies whose culture and processes are not friendly to the idea of updating the designs once development work has started, but ultimately the later a problem is found the worse it's going to be for everyone. Which is why it's suggested to test iteratively through the entire design and development lifecycle. If a company has truly embraced agile or lean development then they won't be concretely fixed to their original plan.
Management / stakeholder involvement
"Management is contemplating why such an involved process was needed
to arrive at this point, proposing to trim down the design process
next time while effectively overlooking the true source of this
newfangled efficiency exhibited by the development team."
I've found this to be the case when management / a stakeholder is not brought along for the ride.
Workshop and testing participation
If it makes sense, and generally it does, insist on management being part of early workshops where ideas are brainstormed, research findings are presented, stories are elaborated etc, so they can see the process for themselves. My assumption here is that you're facilitating these and they'll see it as part of the design team's contribution to providing value, but also that they'll see problems being worked out and that a solution doesn't just 'appear'.
What's really useful is if you conduct some of your usability testing in-house, allow them to observe the testing for themselves so they can see the valuable insights coming out of that process and all the work that goes into a session. This can be in-person if they can help you write notes (don't overwhelm the user though, max 1 additional person), or, remotely via camera/microphone beamed to another room or online.
As pieces of the final product fall into place, it's a good idea to showcase progress to a wider audience including the product owner, any internal users etc. This may or may not directly involve you facilitating this showcase, but it's an important part of avoiding surprises and rework later.
Include not only the work recently developed, but how the research and design effort informed the solution that's been developed.