I would like to be able to change the form to add a new instance so it is more user friendly.
If you need to make your form User-friendly you should use InfoPath to customize it , Check How to: Customize External List Forms Using Microsoft InfoPath
Users will only be able to create new instances, not update or view any others.
You can manage this during configuring the External Content Type by Selecting
- The Create Operation is used by the External List when data is being created. The Create Operation contains the statement allowing data to be inserted into the table.
You also can manage this via permission by creating Permission Level
Only_add and assign this to group and set the users for this group
I would also like for a few of the columns to be a drop down that the choices are in another table
Unfortunately, There are not Lookups fields.
the only thing that you can do a look up on in an external list is on the ID column, anything else and you are out of luck.
See also External List Limitations in SharePoint 2010/2013