If the question was how to get through the air defences... You can either kill the defenders (but it's a crapshoot until you get proper aircraft yourself; still, even a minigunned-up Little Bird can score against a MiG sometimes), bait them away with a sacrificial or just fast decoy, or merely wait until they either drift off or despawn. New air defences take a while to respawn (a week or so?).
The reinforcements spawn at the map edges (or close-ish) away from your starting edge. Do note that this means they can spawn in a corner adjacent to your starting edge, so that might not be as far as you think. Otherwise, they do keep some distance from your agents (at least 10 tiles or so).
But in the end, they still spawn in with full TU, so if your people are close to their spawn area and/or within their newly established line of fire, that's still a bad turn waiting to happen. Tier 3 manors have several waves of reinforcements, which is why they're extra bad.