This plot line has been used a lot in literature, an excellent example being Romeo and Juliet: Juliet takes a substance given to her by the priest that makes her appear, to all intents and purposes, that she is dead, whilst she awaits the exiled Romeo. I have also read a similar story-line in a Wilbur Smith book. In fact, the romance of Valantine and Maximillian draws many parralells with that of Shakespear's star-crossed lovers - but with a very satisfying happy ending.
Dumas does not spell everything out, which is one of the things I love about this book, but the clues are there. Throughout the Count of Monte Cristo, we are drip-fed imformation on the Count's aptitude for chemistry; in the chapter "Toxicology" we learn that he has conversed with Madame de Villefort on the subject. Do not forget that the Abbe Faria is also a skilled chemist, and Dantes, as his protege, learns much; he then has nine years and virtually unlimited funds to build on this knowledge. The Count is also adept at mixing substaces for recreational use, as in the chapter "Sinbad the Sailor", which he carries around in a hollowed out emerald. Furthermore, note his comments to Albert and Franz in "Mazzolata", where he reveals a facination with and in-depth knowledge of death. All this suggests that he would have the ability to create a pill that will slow the heart-rate so it is almost indectectible, still the breath, and even bring on a state similar to rigour mortis.
Whether or not this is actually possible in real life, I do not know. However, my advice would be to suspend your disbelief as the conclusion is well worth it.
(Post is by Laura Dudley not Thomas Wade - we share an account)
My reading of this situation is that the Count decided to simulate Valentine's death in order to further incriminate Mme Villefort, and to push Villefort himself further over the edge of madness. But he (or the author) couldn't decide when to resurrect her - so we have to go through the whole funeral thing, and Maximilien nearly commits suicide etc. In the event he left the reappearance of Valentine far too late: in fact the end of the book is quite badly handled, not only the Valentine episode, but it's also difficult to make any sense of the business with Haydee.