Postponing tevilah to avoid Shabbat without makeup

There is no need to be apologetic or concerned with sounding shallow.  Each woman has her own considerations and concerns, and they are all legitimate. 

It is good to hear that you are receiving an early education about the laws of niddah.  Many kallot feel overwhelmed by the wealth of new information they suddenly have to deal with on the eve of their marriage.  However, B"H when you get married you might find that you rarely go to the mikveh Friday night.  Mikveh night can fall out on six other nights of the week, and lifecycle events such as pregnancy and nursing can keep you away from the mikveh entirely for long periods of time.

You might also discover that getting to the mikveh as early as possible is more important to you to than keeping your eyeliner on or showering.  This is a perspective that might change upon marriage.  If it does not change, then when the time comes there may be room to delay immersion as long as there is mutual agreement between husband and wife.

With the perspective of marriage, and the knowledge that there are solutions available depending on the specific situation once you are married, we hope to hear from you when the question becomes a practical issue, B"H!