Dear Zohrab,
I believe that you changed the type in the MGNCREAT. However, this does not automatically delete the continuation lines present, as you can see in the snapshot below (circle area in the bottom frame).
You can now edit the card with the bottom indicate by the red arrow and set the number of lines to 1 in the Edit Card window, or, even more simply, delete the MGNCREAT and create a new one.
Yes. The constant field is a crude simplification for a solenoid filed.
It depends on your application if it is suitable.
Yes, you can with an RZ interpolated field, which can be defied by the MGNDATA card. More details in the manual.
Note that card can have as many continuation lines as needed by the number of nodes set in the MGNCREAT card.
Please let me know if you need further help.
Cheers Luigi