Busted some bolts removing my battery tray...

You should be able to access the rear of these bolts by simply removing the fenderwell, but it would take quite a bit of bodily distortion. They MIGHT also be covered up by the horn bracket and wiper fluid resevoir, but its been a few days since I took mine apart. You may be able to simply drill new holes and move the tray forward or rearward a half inch or so, I ended up moving my battery to the trunk and threw the tray in the trash, but it shouldn't be too bad for you.

the heads of bolts just LOVE to snap off whenever i put a wrench anywhere near them, so i've gotten pretty good at extracting broken bolts...

one method that works for me is taking a sharp but narrow chisel and hammering it into the body of the bolt, then put a flathead screwdriver in the slit you created and back it out that way

i've found those bolt extractor kits usually just snap as soon as you put any kind of load on them, spray the bolt with some penetrating oil and get creative, worst case scenario drill it out and get a bolt one size larger to fit the larger hole