Additional processing times:

This is a very frustrating part for lots of doctors, especially those coming from Muslim countries. This can potentially harm your career prospects and taxing financially.
The Additional processing time is variable and different for different people. It can take few weeks to months. I suggest not to hide anything from the visa officer, even though if you think it is trivial and does not matter. US embassy can track the records well beyond 12 years (my personal experience).
There is no guarantee if you had additional processing once that you will not have it done again. I applied my visa from London embassy, UK and waited 7 months for the clearance. During this period I could not go for CS exam and hence lost my Match. I applied from my country of residency (Canada) once I thought I had clearance after 4 months of clearance confirmation from State department. I was surprised that I required the processing again. I hope this time it will be only few weeks. I will keep updated.
The key here is that you have to plan really early and anticipate that you will require these additional processing time. You have to give your CS early, may be when you are going for your step 1 or even before that.
NRMP will remove all the FMG's who do not have passed the step 2 CS before certain deadline, usually the last day of December for next years match.

Visa for Clinical Skills Exam

There is no special visa for clinical skills exam. You have to apply for B1/B2 visitor’s visa. Visa officer believe that every applicant for non-immigrant visa is a potential immigrant. Your task to acquire the visa is to prove that you are not a potential immigrant. You do this by showing
Strong ties to your home country. This will be in the form of fixed assets in your home country (i.e. property) and employment commitments (take a letter from you employer confirming your job and pay.
Financial standing: visa officer likes to see that you have enough finances at home that can support your visit and stay during you visit to United States. You can show this in the form of your recent payslips if you are currently employed and your bank statements showing transactions for last 3 to 6 months.
People who are still in the medical school who have financial backing from their parents stands a very good chance to get a visa because they have a very strong reason to come back to their home country to complete there graduation after passing the exam.
Marriage: people who are married stand a better chance than people who are not married.

There is some debate in the fmg’s about disclosing at the time of interview if they are going for CS or just for a visit. I don’t think it is a good idea not to disclose anything. I know lots of fmg's applying from UK feels that they don’t get any problem when they have applied for visit visa for tourism rather than for CS purpose.