Did the Apostles actually witness Our Lady go up to Heaven?

I don't think so.

However, the Church defined the sacred mystery of the Assumption as a divinely revealed dogma of faith.

The Immaculate Mother of God, the ever Virgin Mary, having completed the course of her earthly life, was assumed body and soul into heavenly glory [Pius XII, Munificentissimus Deus 44].

So anyway.

Last night I was wondering why Our Lady permits people to be misled by dubious apparitions many people claim to have.  Why does God allow it?  Why doesn't Our Lady intervene and clear things up, telling us: "That apparition is false.  I never said that."?

There are many reasons, I think.  Chief among them is because it is up to the Church to discern, to approve or disapprove, or reveal there is or is not evidence of the supernatural taking place.  The Church takes Her time, but indicates gradually - in some famous cases - that events may not be supernatural in origin.  The fact simple believers have responded in such a way that they remain faithful, or grow in fidelity to Catholic doctrine and practice, the Church may proceed more cautiously, considering it might do more harm to 'pull up the tares' and risk pulling up the wheat along with them.  That said, even at that point - no fruits have been harvested - good or bad.

A supernatural intervention or sign, clarifying the events or messages are normally commensurate with the phenomena.  That is what happened at Lourdes - instantaneous cures; and at Fatima - the solar miracle and specific prophesies fulfilled.  Perhaps in some cases - the lack of a supernatural clarification as I suggested above, is unnecessary because the Church has the authority, the power on earth to bind and to loose.  Nevertheless, it is faith that God requires, faith is a theological virtue:  Faith in God and what he has revealed in his Son, Jesus Christ, as defined and taught by the Church.

The authority of the Church. 

It always goes back to the Church.  Not this cardinal or that bishop, (except of course, the local ordinary of the place in question) or this priest or that spiritual director of a lone locutionist.  It is the Church:  The Pope and the bishops in communion with him, the Holy See or Magisterium of the Church, or the local Ordinary, has the authority to determine whether or not an apparition or private revelation is worthy of belief.  For clearer information on these matters, read the section below and follow the link at the end of the excerpt:

Responsibility of the Faithful. Today there are a myriad of alleged private revelations and apparitions vying for the attention of the faithful. None have been definitively judged by the Holy See, some have been approved by local authority (e.g. Akita, Cuapa, Betania), others have been found lacking in supernaturality (e.g. Medjugorje, Garabandal), some few have been condemned (e.g. Necedah, Bayside) and finally, the vast majority have received no attention from Church authorities whatsoever.

The first responsibility of the faithful is to remain firmly established in the faith, in the sacraments and in communion with the Pope and bishops. Any Catholic who gives their primary attention to alleged private revelation at the expense of Sacred Scripture, the teaching of the Church (especially the Catechism), sacramental practice, prayer and fidelity to Church authority is off course. The running after spiritual phenomena, such as alleged revelations, is condemned by St. John of the Cross as spiritual avarice. This means that pious souls who would be repulsed by crude materialistic greed think nothing of being greedy to know revelations and prophecies. An exclusive, or even a predominant attention to these matters (especially apocalyptic ones), cannot help but produce an unbalanced spirituality. Should the Church condemn some favorite alleged revelation such a person may find themselves believing more in it than in the supernatural authority of the Church. The devil will have succeeded in what he had set out to do. - Read more here.

I hope I've learned my lesson.  Even holy individuals can be deceived.  I often recall how St. Therese and her community were deceived by Leo Taxil, how St. John Paul II was deceived regarding Fr. Maciel, and how many - even priests - were deceived by Fr. Gino, a false mystic.  Stay clear of websites and news portals which indiscriminately publish promotional material for false visionaries and mystics.

From the same article above, some important points to consider:

• Authority: The Teaching Authority (Magisterium) of the Church alone, therefore, determines what Catholics must believe by this divine and Catholic faith. Everything else in life rests on human faith in the credibility of assertions of truth of one kind or another, such as whether John Wilkes Booth actually shot Abraham Lincoln or whether the Blessed Virgin appeared to a certain person.
• Just because one has a spiritual director:  The witness of prudent priests, especially the spiritual director of the person, is a key element in determining credibility. However, even here care must be taken. The spiritual director himself must be competent in mystical theology, credible as a person and in good standing with the Church. False mystics have been known to "shop" for gullible, extremely aged or incompetent directors. Ideally, a bishop upon hearing of an alleged mystic would assign a competent director, thus insuring the authenticity of the evaluation. 
• If the bishop says no - it means no: With respect to any disciplinary precepts the bishop makes concerning the apparition and its site, they should be followed faithfully (e.g. what sacraments, if any, may be celebrated there). No Catholic should ever violate the practical norms laid down by the local bishop with respect to an alleged apparition, even if intellectually they disagree with his conclusion regarding the alleged apparition. Such disobedience would be sinful, and if it characterized the attitude of the followers of the alleged apparition it would be a sign of its inauthenticity, i.e. by producing bad fruit.

Don't listen to me however - listen to the Church.