First things first, how hard is it to get the bumper off?? I'll need some details on that.

My main issue though, is that I want to seal up my headlights and inverter to prevent any water or moisture from getting down there. I'm only having a problem with my driver side light right now, because a month ago I had to get my whole headlight housing replaced and they did a sloppy job of making a tight seal between the body of the car and the headlight itself. (How can I fix that also?)

I just need a recommended sealant that I can use...

 

Removing the bumper is easy....4 push clips under the bumper, a few on the wing piece under the hood...2 screws in the front of the wheel wells and 2 bolts on the bottom of the bumper. One on the right side, and one on the left side. Then forcefully remove it, but beware that you dont pull too hard because you need to un-hook your fog lights once the bumper is off.
Use oem silicone.

 

there is a diy on resealing the headlights- usually done along with blacking out the internal parts of the housing

you have to take the lens cover off- involves kitchen Oven!!- then clean out old crud and reseal with honda stuff

the ballast prob is when moisture gets inside the light housing and drips down and out thru its vent hole- directly onto the ballast
make sure the lights are tight and the rest will follow

 

Check the DIY section. All of your questions can be answered by taking a look in there. Look for the Lexan fog lense replacement DIY, it should answer your question about re-sealing.