Good luck with that, I use a drop spreader when I need to control the pattern closely. I use a spin spreader in open areas have a larger 3pt and a small cheap tow behind for the riding mower, both of these will throw the heavier granular fertilizer farther than the seeds.
For your uses a small spin spreader throws about a 15=20' swath and might be the best way to go. I use a small cheap Deere poly tow behind about $250 for this. I don't think the larger Cosmo unit is going to work in the narrow confines you mention, I know that my larger 3ph spin spreader throws about a 50' pattern which would be a waste of seed and fertilizers.
As I mentioned a drop spreader would be best for control but I don't think in your case it would be worth the money for limited use. I use mine on short manicured lawns or new clear ground seeding. If the grass is tall it is hard to see the tracks of each pass, so easy to skip or miss spots.