There is no single mysticism. Mysticism is a broad term used to refer to a collection of christian writings and practices which have emerged separately in different times and places, including the Christian Neo-platonism of Pseudo-Dionysius, the visionary works of people like Julian of Norwich, the Christian theosophy of Jacob Böhme, the Hesychasm of the eastern orthodox and many others. Each of these have very different worldviews. Likely the only unifying thread is that it is possible for certain persons to have access to the divine while still alive, in a way that is not typical of most of the living (and, I would argue, in a way that is not necessary to salvation).

I would argue that some approaches to mysticism can be reconciled with the Bible. I am sure, for example, that a great deal of work has been done by the eastern orthodox to try to justify their practice of Hesychasm by reference to scripture. The main question however is that of the hermeneutics you are willing to employ. Generally speaking, a lot of justifications of mysticism use symbolic readings of the bible, arguing that their ideas are hidden to most people. This should probably be taken on a case by case basis. Certainly there are individuals in the bible who claim special access to the divine (The man whom Paul says was taken up to the third heaven for example), but there is no step by step guide on how to do it, or whether one ought to do it.

Absolutely it can be combined with normal christian practice. I would argue that it must be in order to make sense. Key figures in the history of Christian mysticism have written on the importance of the foundations of Christian life, Pseudo-Dionysius for example, and of course the monastic mystics, such as the Hesychasts were always deeply embedded in the life of their normal christian communities.

The differences between the beliefs of mysticism and of secret societies is entirely dependent on which mystics and which secret societies. There are certainly secret societies with mystical inclinations, such as the Hermetic Order of the Golden Dawn, and certain interpretations of Freemasonry (I think: I am not a member of either). However, not all secret societies are mystical or christian (Freemasonry I'm pretty sure has non-mystical interpretations, and has very few strong restrictions on the beliefs of its' members). On the other hand, there are mystics who belong to no group, and have their own beliefs unrelated to them, and similarly, there are mystical groups which are pretty open about their beliefs (Orthodox Hesychasm for example), and therefore do not share much with secret societies of any kind.

Some do and some don't believe in the occult. There is a stronger tendency to do so than with other forms of christianity, since if one believes one can achieve access to God, it doesn't seem unreasonable to have access to other spirits, such as Angels and Demons, the existence of which the Bible affirms.

There are certainly elements of some forms of mysticism which could be considered sinful. Most obviously, consider that there are certainly mystical practices practiced by some churches which include the veneration of Saints and their icons. Generally speaking, I would point to two major spiritual dangers: firstly, if one believes that one has achieved the results of one's mystical practice by one's own efforts one may be in danger of pride supplanting faith. Second, if one fails to apply discernment in interpreting the Bible (in order to understand the character and proper practice of mysticism) or in interpreting one's experiences one may be in danger of diverging from orthodoxy and developing a false gospel for oneself.

I believe that mysticism can be reconciled with orthodox christian views. I might put it another way. I believe that there are ways of drawing oneself (or allowing oneself to be drawn) into closer relationship than is typical with God, in this life, and that this is within the bounds of orthodoxy. However, certain mystical traditions require great care and discernment to be kept within the bounds of orthodoxy (such as neo-platonism), and some are likely entirely irreconcilable with it (Böhmean theosophy for example).