Modern spacecraft have nothing in common with a NAC shell. It's difficult to stress just how fragile anything we can put into orbit today is compared to the flimsiest AeroTech unit.
And it would take 1-2 seconds for a NAC projectile to travel from the top of the atmosphere to the surface. Even with maximum deflection from a 100 km/h wind (for an unrealistic planet where the winds are equally strong all the way up to space) the shot would deviate by ~55 meters. Less than 2 ground hexes.
A WarShip with a big battery of NAC/10s and ground spotters could simply remove any 'Mech or vehicle force that stood in the way of a conquering army.
Would that WarShips had originally been given a factor of 10 less thrust so that maneuvering them into a planetary gravity well was considered an unnecessary risk. That'd keep them out of the ground battle. But it's a bit late now.
You are neglecting the complexities of orbital mechanics.
If something is in orbit and accelerates straight down, radially in, that actually has very little effect on its altitude. It will have a MUCH bigger effect on the eccentricity and the latitude of the apsides of the elliptical orbit. But you won't notice those effects until half an orbit later.
To get down to the surface, an object needs to reduce its orbital velocity, so any projectiles would have to be fired
backwards. How fast they are fired will determine where they land. So if you fire 2 projectiles with different velocities, at the same time, in similar directions, they will hit different locations on the planet.
Picture this, you and a couple buddies are at the range shooting at a target 1,000 feet away. One of you is shooting .308 Winchester, 3,000fps, another has .300 Blackout, 2,000 fps, and the last has .22LR, 1,000fps. I've rounded everything for convenience. If you all squeeze the triggers at the same time, the bullets will all strike at different times.
Now imagine that you are all sitting in the back of a pickup truck, driving around a race track, trying to hit a target 1,000 feet away in the center of the track. It starts to get complicated with having to pull lead on the target to account for the fact that you are in motion, etc. Once you scale up to orbital numbers, you are leading the target by 90 degrees and adjusting the muzzle velocity by adding or subtracting grains of gunpowder.
There's a guy on Youtube, Scott Manley. He's an astronomer by training and has some good videos discussing the weirdness in orbital mechanics.
https://www.youtube.com/watch?v=i5XPFjqPLikhttps://www.youtube.com/watch?v=y0zkhQFHNac