The answer is no, General Finishes Milk Paint does not require sealing, especially for lower use pieces such as a coffee table. General Finishes Milk Paint is a high tech mineral based paint created using modern technology.
Oil and water based finishes do not mimic one another regarding highlighting the figure of the wood. Oil based products will generally achieve more color and definition. However, you can use certain products together to achieve your objective by using oils for color followed by water base finishes for clarity.