Actually, if you have already done extensive clinical training abroad, you can petition the ABR to have the internship requirement waived. Doing an internship in the US system is a good idea bc it allows you to understand the often bizarre workings of a US hospital from the 'other side'. However, if you come from abroad the ability to jump directly into a radiology spot (if one opens up in the middle of the year) can be beneficial.
Yes.
If you are convinced that you will go into IR, a surgical prelim is not a bad idea. You will learn to take care of surgically ill patients, skills that are much more useful later on than the skills you pick up in medical internship.
Most nondesignated surgical prelims are scut-o-ramas with little teaching and excessive time requirements. Many surgical residency programs use their nondesignated prelims to staff the ICUs and other punitive rotations. Some nondesignated surgical prelims are fair and you get the same rotations as the categorical interns or the designated prelims. Make sure you talk to someone who has done a particular prelim before you consider it.