Is saying "the Rebbe is (was?) Mashiach" worse than saying there will be no future Mashiach at all?
Sanhedrin 99a
ר' הילל אומר אין להם משיח לישראל שכבר אכלוהו בימי חזקיה
R. Hillel said: There shall be no Messiah for Israel, because they
have already enjoyed him in the days of Hezekiah. (Soncino
translation)
Was R. Hillel a heretic? Though the Talmud immediately cites R. Yoseph as strongly rejecting R. Hillel's view, it does not say that it is heretical. In fact Rashi there explains R. Yosef's outburst as:
ימחול לו הקב"ה שאמר דברים אשר לא כן
God should forgive him for saying things that are not true.
It seems clear from Rashi that the problem was that the statement was false, not that it was heretical.
Indeed several rishonim (e.g. R. Joseph Albo in the beginning of Sefer Haikkarim, R. Isaiah the Younger in his commentary to Sanhedrin) cite this as a proof against Rambam that disbelief in Mashiach is heretical, because it is inconceivable that R. Hillel was a heretic.
I saw that R. Yehuda Herzl Henkin has a responsum on this question and he cites the same Talmudic passage as evidence that it is not heretical.
Shu"t Benei Vanim 4:26
מה שרבים מחסידיו מדמים שהגאון רממ"ש ז"ל הוא מלך המשיח הוא הבל ושטות
אבל איני יודע איזו מינות ואפיקורסות יש בזה הלא הם מצפים למשיח ומאי
איכפת לן שלדעתם הטועה כבר היה ומת ועתיד לחיות שוב ולמלוך סוף סוף הם
מצפים למשיח שיבוא ועדיפא מרבי הילל במסכת סנהדרין דף צ"ח עמוד ב' שאמר
אין משיח לישראל שכבר אכלוהו בימי חזקיה עכ"ל ולא נידון על כך כאפיקורס
ומה שהם טועים בזהות המשיח אינו אלא כמי שמדמה לעצמו שהמשיח לא יהיה אלא
מי שיש לו שיער אדום וששוקל כך וכך שכל אלה הם דברי הבל ושטות אבל
אפיקורסות אין ולכן בטועים אלה אם הם מקפידים במצוות אין חשש לאכול
משחיטתם ושאר העניינים שהזכיר במכתבו
That which many of his Hasidim imagine that the gaon R. Menachem
Mendel Schneerson of blessed memory is the King Messiah is nonsense
and folly, but I don't know any heresy that is involved in this. Do
they not yearn for the Messiah? So who cares that according to their
mistaken ideas he has already come and died and will in the future
live again and reign? At the end of the day they yearn for the Messiah
to come. And it is better than R. Hillel in tractate Sanhedrin 98b who
said "there is no Messiah for Israel because it was already used up in
the times of Hezekiah", and he was not judged to be a heretic on
account of this. And that which they are mistaken in the identity of
the Messiah is merely like someone who imagines to himself that the
Messiah will only be someone who has red hair or weighs a certain
amount which is certainly nonsense and folly but not heresy.
Therefore, when it comes to these mistakers, if they are careful about
mitzvot there is no concern with eating from their slaughter or any of
the other things that he mentioned in his letter.