@avik there are four conditions to qualify as chezkas Moshiach as per the Rambam:
1) descended from beis Dovid (the Rebbe was)
2) well versed in Torah and toiling in mitzvos like Dovid his father
3) compels all Yisrael to go in the ways of Torah and strengthens it’s breaches
3) fights the wars of Hashem
Then we may with assurance, consider him Moshiach.
If he:
1) succeeds in all the above
2) builds the bhmk
3) does kibutz golios
Then he is indeed Moshiach and then he will get the entire world to serve Hashem together.
The Rebbe definitely fulfilled the first four: he started the momentous baal teshuva movement as waged the wars of Hashem against assimilation the continuation of which is ongoing by his over 5000 shluchim. And all it takes to qualify is sincere effort, not complete success as that is the first of the conditions for Moshiach vadai