What it comes down to in the Balkans is that taking either the Italians or the Soviets for an ally for a guarantee of Security against the other is poisoned, though in inverse to the distance Both Hungary and Romania and maybe Bulgaria can handle the poison of an Italian alliance, Greece and Yugoslavia can maybe handle the Soviets and Turkey is just screwed. France is viewed as non poisonous by pretty much everyone (except maybe Turkey) and the British are even better.
In regards to an attack from a previous Neutral...
The following Neutrals border the 3R and its conquests:
Sweden (/Finland), USSR, Romania, Slovakia, Hungary, Yugoslavia and Italy.
The only way to get Entente Troops to Sweden is on the railroad and I don't think the shipping volume is there to make it work. The USSR doesn't need or want help. The Romanian border is *tiny* though they would be happy to host the Entente Troops, the Slovakians would probably could be overrun before they get things started. The Hungarians are a possibility, but like the Yugoslavs and the Italians, any attack would be through mountainous terrain. The area of attack into the 3R where the terrain helps the 3R the *least* is on the easternmost end of the current British position.