Is the display replaceable in a 3d led TV in general? My specific model is LG 55LM4700. I've looked online for the replacement part but haven't found it. The problem is black bands several inches wide going horizontal and vertical from the point at which the screen suffered impact damage from a projectile.

Replace the jack wires going to the LED strips. Or look for the one bad jack wire going to the LED's and fix it. Use a volt meter on ohms to find the bad wire.

Yes, it can be fixed but you have multiple problems. By the time you replace all that has to be you would be better off buying another disposable TV.

Back - light problem. Check and replace damaged component/s at its back-light inverter board, or replace this board as card basis. If you wish to get some details; check the site linked here. Surf the site with patience. Pull up older posts. It will be best to replace the inverter board as card basis.
http://electro-medical.blogspot.com/
To some model sets; especially LED back-lighted types; the back-light inverter circuit will be part of the main power supply board [SMPS]. In such cases, the power supply board might need replacement. To some models sets, the back-light inverter and SMPS section circuit will be integrated at its main board itself. If so; the main board should be replaced. To some models of sets, all these section circuit will be integrated at its main board. If this is the case, you have to replace the main board.

Most likely your back light or your inverter.

it sounds like a blown capacitor in the unit somewhere.
here are some tools you are going to have to have: ohm meter,oscilloscope, signal generator, soldering equipment,
grounding equipment, diagrams, schematics, test points, layout of circuits,

replace backlight inverter

You cannot replace a screen that big. The manufacturers wont ship it to you. IT has to be done by an authorized service center.

It depends on the specific LG TV model.
Most LG "Smart" TVs have some way to view photos and watch videos from a USB device. In general, the user interface will pop up when the device is inserted.

Unfortunately, the embedded firmware supports a very limited number of video formats (codecs) since LG needs to negotiate for and pay licensing fees for each codec. Since most TV buyers prefer a low price to a rich collection of codecs, there are a limited number of supported formats.
The technically minded may find LG Smart TV Document useful.

1) you must try again, replace the batteries with new one.
2) if doesnot work call the LG ,company go www.lg.com and get support online via chat, and ask them about your problem.
Most of the time is the Microcontroller section, for VCC +RST+CLOCK problem, this is a difficult for normal persons to test, you must go to the repair shop TV as another option,if the LG company fail.
God bless you