Electrical – Splicing multiple LED lights

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.

What you want is a ceiling box that uses an old-work brace

If you have ever been in a hardware/home-improvement store and wondered why some of the boxes had what looked like a long metal brace attached to them, like so (picture for illustration only):

wonder no further. That brace is meant to go between adjacent ceiling joists and support both the box and a fixture attached to the box, and the box can be adjusted to position it in the joist bay. The fixture itself then acts as the cover for the box, as per NEC 314.25 and 410.22.

(Trying to support a box using a lighting fixture would likely be seen as a NEC 314.23 violation -- there is no subsection of 314.23 that allows for that.)