If you are already entering data via a separate data entry table then I would suggest you manage data validation via scripting, not via field schema. To keep it idiot proof, you would also need to prevent users from creating records directly in the main data table, which you can do via account privileges, custom menus, etc.
As far as validation within your script is concerned, you can generally set this up with better user communication than using FM's field validation anyhow. You can use Conditional Formatting on the data entry layout to visually communicate to the user fields that must have data, or even have particular types of data. And the script would specifically check each field against whatever validation criteria you want to use and spit the user out—with an appropriate message—if validation fails.
If your user is in the middle of a scripted data entry process, you can have script triggers on fields that will run validation subscripts and then return to the main scripted process.