Can I convert a 12 volt DC device that uses a 240v transformer, back to 12 volt?
You can just remove the transformer and wire the devices direct to the 12V circuit (paying attention to polarity of course).

Current is drawn by devices, and your power supply must be able to support it. There's nothing you need to 'do' other than ensure your supply can provide the current your devices need.

Note that a 50W panel at 12V is 4.16A, so it may not charge the battery fast enough for your situation -- even if you're only using the 5.2A lights (as opposed to the full 10.4A load with the lights and pump running).

This may or may not be a problem depending on what you're doing. If your battery is rated at 100 AH (amp hours), and you're not charging it, it should in theory power the LED lights for about 19 hours (calculator). This isn't exact because battery capacity can be affected by load, charge level, and other factors.