So, you do hold open the possibility that Lazarus was assumed into heaven.
Yes, I think that's fine and the same can be said with Mary. However (like I've always pointed out), you can't rule these as doctrinally necessary to bind upon all Christians. It's 100% speculation, and the Church knows this.
There is absolutely no tradition of Lazarus being assumed into heaven, but there is 2000 years of tradition of Mary being assumed into heaven.
Do you really believe that Christ would let His Body wander in error for 2000 years?
He let the entire nation of Israel live without a king for some time prior to Saul. Each man did what was right in his own eyes. Why must you have a central king/leader to tell you what to believe?
It's not about what you or I think that "must" be, its about what Christ did
Jesus called Simon "the Rock upon which I will build My Church."
Why would Jesus call Simon by a name that was reserved for God, what is the significance of it, and what does that say about Peter's new mission?
And Jesus said that, "I will be with you always, unto the close of the age."
Either Jesus knew that His Church would go into error, knew it, and refused to protect it(which would make Him a liar), or He said it while not knowing that His Church would fall into error(in which He was deceived), or that He was powerless to keep the promises that he made; any of these options would suggest that Jesus was not divine and therefore your faith in Him is utterly useless.
We do not. Hence, a tradition held by all Christians for so long a time can be seen as nothing other than an apostolic teaching.
So a folk tale is considered an apostolic tradition?
Mary's assumption is no less factual than that "folk tale" known as the Exodus is considered historical fact. There is just as much traditional attestation for one as to the other. So to reject one then, that is if you are logically consistent, means that you must reject the other.