Not exactly sure, maybe physwizz confirm, I presume, you tried install kernel file by Install button in TWRP. I tried it too, failed it too and
a) you have done backup in TWRP, thats usual reason to setup TWRP - you just in case backup those special partition and if you done it, restore boot from backup will help
b) really not sure about that, but Odin, button BL and flash BL file from original stock ROM.
Can anybody advanced confirm? Maybe it can be helpful to know exactly, what for sure are kernel (AFAIK part of boot partition), "bootloader" and "boot" partion, BL button in Odin
must be from the recovery ap section