It depends

It's too tight to get an appointment in Canada or Mexico now, epsecially in Summer. Maybe you should try it in China. However, you should tell your employer very frankly about the possible problems for doing that.

Security check or not may depend on what do you do at work. Is it so called sensitive area ? This is what you really need to tell your employer upfront. Regarding visa application, you'd better prepare yourself a good package to ensure your success. If you had a lawyer to help you get your H1B, consult the lawyer about this.