Hey there over the pond.
Mark,
I've breifly looked your project over and it looks like if you add a field for apt status and set it to occupied or available and when your listing apts limit the list to only those that are available.
This will require another interface somewhere for managing the status's but I think it should solve your problem.
Also some of the fields that you are displaying in the subforms look like duplicate data from the main table and there is no need to displey them in the subform. As long as the links for child and master are inplace you don't have to worry about the relationship.
Keep at it.