I agree with you - it was Brody's home first!!!

As far as the room goes, how often will it REALLY be used as a guest room? We have one, and it hardly gets used.

I don't have a suggestion re the window, unless there is a way to install a cat flap into the window frame - but that would inlvove a lot of work I suppose

Re the dirt on the wall - my first suggestion would be to paint that section with an enamel or silk paint, so it can be wiped clean. If you don't have wallpaper, then a cost-effective option is a strip of sticky-back-plastic on the area where the footprints are an issue - as it can be easily wiped clean. (Can't do this over wallpaper as it will pulll the wallpaper off if you ever decide to remove the plastic. Basically anything that gives a wipe-clean surface will solve the problem - I have a thin sheet of perspex screwed to the wall behind the litter boxes as Connor's aim is off sometimes.

Does Brody wear a collar? If so, then you could install a cat flap with a magnetic clip, which is released by a magnet on Brody's collar. So then only Brody would be able to get in and out, and not Tobey. (My friend has one and it works very well - she feeds strays in her garden, but only HER cat can get in through the cat flap).

Good luck sorting this out - I HATE it when I am expected to compromise my cat's comfort to keep hoomans happy!!!