and you can get that letter from your own business as well - as mentioned min salary and 5% for sso has not changed. What changed is, that immigration / labour check if these sso payments has been made and paid.
to apply for non b visa is at an embassy or consulte outside the country. does your accountant / employer prepares the documents for you, that you can receive the needed documents from labour to apply for non-b ?
there are start asking sometimes after 4-5-6 arrivals - yes ---- by law there is no limit, but if you come often, they want you to get a real visa and not come by excemption.