Most likely your direct clutch has damage. The O/D band doesn't usually go out, its the direct clutch that spins making it seem like O/D isn't engaging. However, when you put it in 3rd, you have the forward + direct clutch engaged. I'll conjecture that the direct clutch apply pressure increases when its in 3rd which restores 3rd gear. I've had this happen in one of mine.
Its likely that you have some forward clutch burn up too but not as much.
As for the fluid.. you have mostly toasted clutch friction and some metal particles in the fluid, but the filter catches the most of it. What gets past is very small and just sort of circulates around. Given enough mileage, it could increase wear in the valve body.
If it were me, I'd leave it as is until overhaul time since an overhaul is going to rebuild all the clutch packs anyway. If you can stay in 1,2 you're probably better off. That should help preserve the steel plates in the direct clutch pack too so that you may only need new friction plates. Try to drive it as little as needed until then. If you need 1000s of miles, then its probably a good idea to go ahead and change the fluid out to reduce the amount of microscopic particles. Don't forget the converter.. it holds about 1 gallon of fluid itself.