And if the MM wanted to "reclaim" the Ground they would have to send at least 40 soldiers to guard a place where they build their camp, meanign 40 people would have to go through the forest
what camp? Just because Mountain guys can now live on the surface, doesn't obligate them to all run out start building their village in middle of the forest. Point is, all the damage inflicted on them, is damage they have claimed to be able to repair. Don't know their timelines, but within a few years at most of the "peace" Lexa gave them, nothing prevents them from rebuilding their generators, gas tanks, firing up the gas defense system, shoring up access to the dam... All that work has been done before under the restraints of radioactivity, and will now go much faster with past experience and no restraints.
End result - Mountain is back to being an effectively impregnable fortress, from which Grounders can now be again attacked at leisure. Except where previously Mountain people had some incentive not to exterminate Grounder population and no use for lands they couldn't hold, they now don't. So rather then starting with sending "40 guys to defend some camp on the surface", it will be more like them deploying chemical bombs against grounder villages, exterminating the cattle, setting up defensive barriers/mines, expanding surveillance networks at leisure.
As for 2 scouts stopping the 100, please keep in mind that Mountain military hasn't had problems with hunting grounders and setting up cameras in the forest before, they wouldn't start now. Apparently from what the series has shown us, their soldiers are just as good as grounders in the forest, only unlike grounders they have much better tech.
Even if Mountain soldiers for some reason spontaneously have lost all the same skills which allowed them to (1) maintain hidden camera network around all grounder villages and routes (2) deploy sniper teams to assassinate enemy leaders (3) deploy spotters that went through gronder 'security' with easy, and by his lonesome pinned down half a village of them, (4) hunt grounders - even if they currently get all their blood via reapers hunting for them, someone had to hunt down the first stock for reapers.... and all of that in cumbersome fully body suits, and with oxygen tank restrictions.
... so even if for some reason those skills just evaporated and Mountain military suddenly became shit at a waging a way they have been winning for a century, those "2 scout", hide in bushes tricks don't work so well against an enemy with tech level capable of deploying night/heat vision goggles which, given the other tech shown by the mountain people, they should be easily capable of.