The support of a mirror to resist flex in it, much of this has to do with the thickness of the mirror itself. Your 10" mirror is very thick. 3 point support for such a thick 10" mirror would be fine. Six points is better. Nine points helps too as Glen mentions. Only thing is you don't gain more support for
THIS mirror by putting more support points in. Such a massive mirror does not require so many points. It won't hurt it. Only thing you are creating is more work for yourself.
So how many points? In your case, work out how you will build the cell, and how complicated it will be to make each support system. You are fortunate to have such a thick mirror here as all three support systems will work equally well. My suggestion for you is to make things simple here
. You won't gain anything with complicating matters here. But, but, you will gain invaluble experience by making a 6 or 9 point system in understanding the guts of Newtonian design.
Don't glue the mirror in place unless the whole of the cell is waterproof! Every now and then you will need to clean the mirror, and you don't want to compromise the cell by soaking it.
With your concerns about edge support, two things: 1, don't make it point contact, spread the load over an area. You have a massive thick lump of glass to spread the load over - take advantage of this. 2, the silicone will flex when the mirror is tilted during use. This will see the mirror move ever so slightly, especially it being so heavy, and enough to alter collimation. You need to support its weight. And if you go for 6 or 9 point supports, these will have more movement in them than a 3 point. You are asking for trouble with mirror shift if you don't support the mirror from the sides. Gluing the mirror will work, but it is sloppy workmanship and makes for a compromised mirror.