cr housing units + evr improvements

Hi guys

Our idea is: We need to improve the shelter management (Configurable).
A shelter must be considered as a "container". Housing units must be defined within all shelters. A housing unit is part of a shelter and we can enroll evacuees in them.
The housing unit management guarantees a detailed management of shelters and evacuees.

We  don't care about the "type" of housing unit: 
If the shelter is a stadium -> housing unit is a tent
if the shelter is a hotel -> housing unit is a room etc
We care just what kind of evacuee could be accommodated.

I added improvements in evr module: added filters, added field descriptions, added report fields 

I can simply delete this function editing the  cr_shelter_registration table but I need help.

In the cr_shelter_registration table I added a drop-down cr_housing_unit_id() field. At this time in the registration form this added field shows all housing units defined in all shelters (This is not really useful), can we show just the housing units defined in the shelter I want to register my evacuees in? 

The problem I met is (You have to consider I refer to the evacuees registration tab in the Evr module, so I have to specify both, shelter and housing unit where I am gong to register the evacuee): shelter_id() field and the housing_unit() field are on the same registration form so I can't filter the housing units because I don't know the chosen shelter_id yet!
Does it makes sense?

Is it possible to edit the shelter registration form adding the housing unit selection field after I specified the shelter? Resuming: step1) I select the shelter step2) the housing unit field appears and it contains only the housing units related to the chosen shelter
My idea is something similar than the "Verify Password" field in the user registration form, It appears after I inserted my password...
How to do this? Or any other idea to filter the housing units?