$FFDD-$FFE5 is not RAM. It is part of the ROM area at $8000-$FFFF. On this particular case of Metal Gear, it uses a mapper so that when writing back to $8000-$FFFF it will bankswitch (mapper #2 or UNROM board).
To avoid bus conflicts however the ROM has to contian the same vaules as the one used for bankwitching, so that to switch bank #0 you have to write $00 to a ROM location already containing $00, same for bank 1, 2,... until 6 or 7. (Bank #7 is already present at $C000-$FFFF so there's no reason to swap a second copy of it at $8000-$BFFF). There's various ways of handling this, and apparently the game has a table with the values 0, 1, 2, 3, 4, 5, 6 ,7 at this particular location you mentionned.
$5115 is an unused location except if you use MMC5. However you cannot change from mapper #2 to MMC5 just by changing the bankswitch code, MMC5 needs a lot of complex initialization, in particular to set the appropriate PRG and CHR switching mode, the appropriate nametable mirroring mode, and to disable some features such as IRQs, sound, etc...
I do not recommand mapper hacking the game to use MMC5 mapper unless you're actually going to use MMC5 features.