Your first question is a bit of a stumper without knowing more about your space.  If you like, post or email me a diagram, and I'll take a looksee.

As for a tomb reveal, well, if ya don't have a trap door, you don't have a trap door, and we haven't quite figured out how to fold space yet. here's some options, varied complexities:
 
a. if the directors amenable, create it out of blocks of foam that are dragged/carried onstage and stacked/assembled by the cast.  Think "slaves building the pyramids."  If done right, it could be quite moving.

b. If one of your platforms is tall enough, you might want to consider putting it on casters and spinning it around to reveal the backside as the tomb;

c. create  a third  platform -- the tomb, and throw them all on casters. keep the tomb to the back and center, and the appropriate time, move the front platforms to the sides enough to reveal the tomb and move it forward;

d. create one of your platforms in peices, one of which being the tomb, that you can spin, turn or otherwise "break out" of the platform as a whole to reveal.

e. make sliding front "walls" on your platforms that can open to reveal the tomb.

e. plywood oragami cube.  hinged top and sides. Could have a fabric back, as well if you don't want it see-through. I've drawn this one out, as it would be hard to explain:


 it doesn't need to be thick plywood, it's not supporting weight.  if a permanent part of the stage, it would look like a slight step or a bump (build slight ramps up it so that actors don't trip).  Or, put in on casters, hide it under something and wheel it out in its collapsed position.  As with the possibility of building a tomb onstage, use a display of "heavy lifting" perhaps to erect it with dramatic effect.  Would likely need folks to continue holding it up, but it's a short scene and a fade-to-black anyhoo every time I've seen it, and the peice wouldn't be heavy, really.  [I've never built this, but in my head it works.  If you try it out, please let me know how it goes!]

No trouble at all -- I doodle pretty quick-like, and it's good mental exercise for me, too. :)