Yeah your landpad, floorpad, etc are very confusing to follow.
Are your levels largely static? A lot of iso games will create another texture map of the level defining the walkable paths - whether this is a feasible solution depends on if you are rendering a 3d world from a certain view (and if this view is changeable) or if you are using static textures for your levels, or if you are doing a tile based game.
Isometric tile based (that use 2d sprites etc) benefit from a grid system - you can then predefine different height levels for the grid. IE allow 16 different elevations which would create a grid that is the level's width by height by 16 tall. You then put all of your game objects in one of the grid spaces, and you draw them in an order that gives isometric appearance (usually start at bottom layer at upper right corner and finish on top layer at lower left corner). In this type of grid system you can give each of your objects a height - so that a tree that is 10 layers tall, for example, will not be overdrawn unless some object is at level 11 or higher.
For collision detection, you check each one of your game's dynamic objects against the tiles that it might occupy on completing the move - if something else occupies those tiles then the check fails. Usually you don't have too many dynamic objects (characters, enemies) and so this check is not very expensive.
In any case, isometric with elevation has definitely been done before - just google and you can find lots of different implementations.