For your firmware, you can go to the online configurator for Repeiter and tweak your configuration.h file then reflash the firmware or you can find “# define MAXTEMP [temp]” & “# define UI_SET_MAX_EXTRUDER_TEMP [temp]” and change the values to 285, you may have to go farther down the code to catch 2 other notes for temp labeled similarly (used ctrl+f on the temp in notepad++). I set mine to 280 and never messed with it again and did the hot set off that, haven’t had an issue with anything yet.
I can provide the updated firmware I am currently on if you would like however if there are variations in the build it may still cause a few issues. you can use the configuration.h that you currently have and load it to the new configurator and download a whole new firmware package after making adjustments, just don’t forget to replace the repetier logo with the beast logo to keep the beast flair
As for you’re temperature sensor question, unless the conversion is done in the firmware for F, I would assume its reading low. If could be that it is not making a good enough connection to the block to get an accurate reading but being that its a V6 I would assume its the thermister in the metal tube secured in the heat block like mine is and its just not allowed to get hot enough through the firmware.
Heat creep shouldn’t be nearly an issue with the V6 as long as you have the fan on the cooling tower.
Hope this helps,