I think that what David is saying is the the user knows the the Person, they just want to enter type it in and not select from a drop down.
My suggestion is this:
1. Ensure that the key field of the Person table is the person ID.
2. You already have a relationship based on a normal lookup field which is a drop down list. leave that in place, but get rid of the look up fields on it.
3. Make a new field which is just a numeric field called [Person ID (direct Entry)].
4. Make a formula field called [Related Person (final)] withe formula
IF([[Related Person] > 0, [Related Person], [Person ID (direct Entry)])
Then use that field in a relationship and look up your fields from the Person table.
The effect will be that the user can use either method to get the lookup connected to the Person.
