OOB Impassible territories should in every case be able to be flown over.
Allies should be able to invade neutrals : precedent- US invaded Azores which was previously under Spains control. UK seized Iraq when it had a pro axis leader in power but was still neutral. UK also seized Iceland from Denmark and USA/UK seized Greenland from Denmark, UK seized Madagascar from Vichy France, UK attacked Dakar which was also Vichy. There are more cases.
Soviets also tried to attack Finland and also occupied part of Rumania and took over the independent Baltic states as well as eastern poland. They had semi control of Mongolia as a satellite nation.
ON the Mongolia situation: It can be included as a semi-restrictive territory due largely for its inhospitable terrain features. Ideas:1) only x amount of units can pass into or out per turn ( this idea can be used for Sahara as well) 2) all movement is at the rate of one per turn ( no blitz with tanks) 3) completely make it impassible like the Himalayans