No, you're not making any sense at all - basically, all you are asking for is that you type in the text when creating the form, and then during the creation of the form you are saying it is read only to the person who is opening the form. That was the previous one I explained.
If they can not change the default text, then what is the purpose of typing into the field?
To me the purpose of the form itself is not making any sense at all. If you type in what you want the "default" text to say - you do that when creating the form. If you don't change the properties, it will show what is in the text field - they can type over it, but if they open the form again, it will still be there. It won't be deleted.
If they are using Acrobat reader, then they have to save a separate copy anyhow.
Those are the best ways to control a form in Acrobat when it comes to fields and other aspects of the form. However, if I have Acrobat Pro and you have no permissions (security) put on there - I can change anything I want.