You can also chamfer the edge of the hardwood. Probably easier than chamfering the marble unless it is a small chamfer. A larger chamfer over a portion of the threshold is common for differences as small as 1/4 inch. The threshold is simply turned into a small ramp to ease the transition and eliminate any edges.

They still make a 5/8" T&G plywood subfloor, Lowe's item #12250. You are a little confused about the minimums for a plywood substrate. The minimums are this for joisting 16" OC. If installing tile directly to plywood the minimums are the subfloor (bottom) layer to be 19/32" T&G subfloor rated plywood with a layer of 15/32" sq.edge plywood underlayment installed on top... equaling 1 1/8". This is for residential, light commercial settings state both layers to be a min. of 19/32" thick.... equaling 1 1/4" thick. If you are incorporating either CBU backerboard or Ditra membrane between the tile and plywood the minimums drop down to a "single" layer subfloor using a minimum of 19/32" T&G subfloor rated plywood. Those are the absolute minimums as far as plywood substrates go. I personally go thicker in both cases. Bottom line is 3/4" (23/32) T&G subfloor rated plywood is OK to use underneath CBU or Ditra. These minimums are straight from the Tile Council of North America 2007 Edition Handbook.

I think I can give you a few options to solve your problems. First, try a different underlayment. Schluter DITRA (http://www.schluter.com/5372.aspx) could be a solution to part of your issue, although it is more expensive than concrete backer-board, it's also much thinner and easier to install. Next transition down from the height of the ceramic to the height of the laminate with the schluter Reno-U or the Reno V. I recently did a ceramic to laminate transition in a friends house using the Reno-U and it came out great. Finally, as for installing over the sheet vinyl, it's probably ok to do, because you mortar the backer down AND nail or screw it down afterward. Although personally, I'd probably still tear up the vinyl. I hope this helps, let me know if you have any more questions. Here is a link that might be useful: schluter transitions

Ceramic tile is a mixture of clays and other natural materials. The special clays are mined from the earth, shaped, colored and than fired in kilns. Traditional ceramic tile can be naturally colored and left unglazed like terra cotta, or they can feature colored or highly designed surfaces which can be glazed (finished with a glass surface) either in a high gloss or matte finish. Most ceramic tile has either a white or red body coloration underneath the glazed, colored top layer. Glazed porcelain has a body made of the porcelain material and then a glaze is applied over the surface much like a regular ceramic tile Porcelain tile is a newer form of ceramic tile and extremly popular among homeowners. Porcelain tiles are composed of fine porcelain clays and fired at much higher temperatures than ceramic tiles. This process makes porcelain tile more dense, less porous, much harder and less prone to moisture and stain absorption than ceramic tiles. For these reasons, most porcelain tiles are suitable for both indoor and outdoor installations. Porcelain tiles are also harder to cut due to their density and hardness. For other information you can check this site Ciciliot