I also understand that you can change fields on a layout only if contained in the main table not in related ones.
Actually you can change them. They can be treated in many ways just like a field defined in the layout's table.
How do I lock table 1 ?
For starters, see "Editing record access privileges" in FileMaker Help and check out this particular sub section: "Entering a formula for limiting access on a record-by-record basis".
The trick is to set up an expression where you can set a "lock" on a table and then later "unlock" it.
Define a relationship like this:
Table 1::anyfield X LockTable::anyfield
You create a single record in LockTable. Define a text field, LockStatus in it. On a locktable absed layout where you want to control the lock status, place LockStatus on the layout with a Locked, Unlocked radio button format.
Now you can use this expression as the record access expression in Manage | Security... to control access to the table:
LockTable::LockStatus = "UnLocked"
I have a similar need, but I want table 1 to be locked/unlocked by a script step. Can this be done?