Setting up a separate entity under the main corporation

I would strongly recommend that you contact an attorney to advise you. The best way to proceed really depends on the goals and rationale. It appears, based on your facts, that you are separating part of the value to this particular shareholder or to separate this particular business. There may be better ways to do this depending on the goals and facts.

As to your question, if this corporation is 100% owned by one shareholder they will have absolute discretion on the corporation actions unless you have some voting trust or other arrangement. If you have no problem in having no control, then you can do so. If you want to have control then you either need to have shares or have a separate agreement.

Finally, as to the EIN you cannot use a same EIN for a new entity. Hope this helps and contact an attorney to provide advice on the legal and tax matters related to your venture.