Can I have a text entry customer-lookup and a related-customer drop-down in the same table regarding the same relationship?

There will be a regular field to choose the drop down choice as with any normal relationship.

There will be a new field which is just a pure data entry field where the use can enter an account # manually.

Then there will be a 3rd field which calculated which one of those two fields to use.  That last field will also be used in the relationship - a 2nd relationship to the Accounts table.  All you lookups and summary fields will come off that last relationship.

Please post the formula for 2nd relationship.