Deorbit hardware isn't big, just a small solid rocket motor would supply most of the thrust, say 100kg thruster for a 5 ton projectile. The deorbit burn is only 100m/s or so. That's some very sensitive monitoring for what amounts to ISS station keeping burns. Monitoring effectiveness could be increased by only tracking oblong objects, but such a burn on the day side might be near impossible to see anyway. This is for the LEO type the US air force is interested in.

A higher orbit projectile system would be slower but more powerful, and wouldn't need more than 3km/s to deorbit, so 4 ton-ish propulsion section (an eccentric orbit could reduce this significantly, but would narrow possible targets. The long period could allow ion engines, but the downside is big solar panels). At 30,000 km high anywhere in the sky, that's a lot of high-power telescopes tracking a lot of sky for just an exhaust plume.

Decoys would only be useful for the burns, and possibly only for false alarms. If you know a projectile is coming, you probably have a good idea about it's target, so moving to a different bunker could be good enough. In the same way, if an actual threat exists out there, a decoy burn could spur movement.

I don't think decoy satellites are useful here. If you can track these projectiles closely enough to detect plumes or small velocity changes, no amount of decoys will be enough, and orbital warfare is an entirely different ballpark.

About countermeasures, trying to intercept outside of the atmosphere requires a suborbital capable missile (probably fully orbit capable for an intercept from MEO), which will be huge and would require an incredibly precise final stage and a convenient launch location to have any chance of hitting. If you have that capability, you could just hit the projectile before it's used at all, but again, that's orbital warfare.

As for atmospheric countermeasures, a LEO type will spend maybe 20 seconds in atmosphere, mostly covered with a ball of plasma, so tracking could be a non-issue, depending on method. The issue is hitting with enough force to do anything about it. Most interceptor weapons are designed for much weaker, much much slower targets, and anything short of a direct hit will do nothing. A MEO type will be even faster, with less than 10 seconds of plasma and moving over 8 km every second. Good luck hitting that.