The Asset Custom Fields are tied to specific Asset Types, not to specific Asset records.
So, Any given Asset Type can be set to show ALL custom asset fields or you can choose to have some Asset Types only show specific Asset Custom Fields if desired.
I see the same behavior when i set it up like you have it (asset no., model, location, status set as required fields); when i save only asset no., model, and location are bolded even though it won't let you save unless the other fields are populated with data. I would consider this a minor bug; you may want to open a support ticket about it.
I'm not sure what would cause the accounts in WHD to be marked inactive without any change in AD. I can see where you're coming from on the timeout - if the LDAP connection (which is usually pretty fast) timed out, WHD would essentially see that as "well, i didn't see that record this time" and it might deactivate it because of that.
What I have seen in our case with custom asset fields is that even with an asset type set to show specific fields and none of the specific fields are shown, it shows them all. Ive also seen no way to take a "desktop" asset type and exclude specific custom fields. For example I have Battery Replacement date as a custom field to track UPS battery replacement date. If I create a desktop, laptop, software, etc. asset it shows up. Now setting that specific asset type to only use specific asset fields doesnt allow you to select the default asset fields that you would want to include such as host name or barcode/ID.