Rather than prompt for a SiteID you could create a ComboBox that listed all active SiteIDs. (Don't allow edit or addition to the list.) Create a query on TempHomeSites with a criterion on SiteID that used the ComboBox value to select the appropriate record. The OnChange event of the ComboBox can then trigger opening a form bound to the query created above. Edit data and save as required.
How the database manager gets different action options may be a challenge. Access's security model is not know for its strengths. One option is to split the application into a front end with linked tables and a back end with the data. If the Switchboard (Access's menu creation scheme) is in the front end you can make a separate front end for the manager. The back end goes onto some shared network space. The front end on to users' desktop systems.
The manager's option could use a similar form to that described above but with different actions assigned. The resulting TempHomeSites form could have an action assigned to add data to the MasterHomeSites table.
The best thing is to get started. Everything I know about Access I learned because I had to solve some problem like the one you describe. Expect frustration. Perseverance will get you where you want to go. Good luck.