"1) It seems pretty solid right now, do I need more bracing? For example do I need to put anything on the top bottom panel centers?"

You would probably be fine as things are, but if you want even more solid, easy and do-able with what's at hand, just use the same stock you used for the other braces. Place them so the 1" edge is your glue joint from front to back along the middle of the top/bottom. (but check that they are not curled or bowed at all, since the long dimension will be the major joint) and cut them so they are just snug when tapped into position.

"2) Time to make it air tight. What do you guys recommend to seal the insides of the mdf panel corners? How about pl-400 or one of those window sealers? What have you guys used successfully?"

If you joints and glue job went well you should be ready for sound, but I have seen people use more glue, caulk or what have you for insurance and it is probably all good. I went to no special pains with mine and can attest there is no leaking or air noise of any kind.

"3) What do you recommend I seal all the removeable parts with?"

I didn't, except on my cheap-o terminal cups where I used rope caulk. no problems.

"4) What is a good way to mount all the electronics in the box, worried about vibrations..."

This is where I went for it. I used pegboard as a surrogate circuit board to create my crossover. This gave me ample holes where I used fat nylon standoffs and rubber grommet or rubber washer sandwich to isolate the crossover panel from the cabinet. Just make sure that the screws do not touch the crossover board and that only the rubber does(stack like so, screwhead, big washer, rubber, crossover board, rubber, big washer, standoff). This is probably a bit tweaky, but I like how easy it is to remove since it it not glued down and the standoffs make screw insertion easy enough to install/uninstall blind(assuming you glue all of the below the board parts to the cabinet so the stack doesn't fall apart when you remove it). Got it all at the hardware store cheap.

"5) What is the best way to attach the r13 to the mdf."

I just used adhesive applied in a zig zag pattern. I wondered, but it stayed put. A few staples to keep it down while it dried was my insurance and I couldn't tell you which holds it now, but it is very there. Note, if you use staples, few is good, many will tend to keep the glass squashed down in so many places and it doesn't suck up sound so good. Fluffy glass = happy box.