Sounds like something someone would have noticed a long time ago.
Only if they had taken the time to make accurate measurements at a range of different locations.
Yeah, but the place you tend to find hostile ships is also in space. They had no reason to suspect an entire ground-based base
Non-hyperdrive capable tie fighters in the system without a capital ship gives every reason to suspect a ground based base. And even allowing that Din and Bo-katan are too dim to work that one out, they have no idea what may be on the planet's surface, based on a brief expedition to a single location. With a military expedition, you do not go in blind. You assume the presence of hostiles until proven otherwise. And if the manadorians had believed the surface to be safe, they would not have sent their leader and best warriors on a simple recon.
Basically, mandalorians are like klingons (see Star Trek III). They charge in recklessly, hoping they do run into trouble so they can prove how badass they are.
, and any ships coming in from space wouldn't detect the landing party, because of all the interference.
This is true, but any hostile ships "just passing through" would need to be dealt with sooner or later if the planet is to be reclaimed, and anyway, if you are in space and faced with a strong hostile force you can run away.
And you're deflecting. The point stands that this is a time-sensitive mission.
Really, no. Not even close. They could (and should) have spent
years on Nevarro planning and preparing.
But the mandalorians would have got bored and wandered off. Because that is the way.