If you omit it, you say that all names are long. So here is better to use "ta" to specify that you speak about some concrete names.
I believe the word for "these" is a different demonstrative that comes later. (in this instance I think "these" would be tato)
You are correct.
In the related "To jsou..." we would also accept "These are..." as a valid translation, but that is syntactically different.
Because in "Those are long names" long is an adjectives directly modifying names. "To jsou dlouhá jména."
In "Those names are long." you have "those names" and you are telling they are long using "are long", "Ta jména jsou dlouhá.".
These two different syntactical forms are distinguished throughout this course.