Yes, you can take the aptitude test as often as you want. The game will always use the last result when it determines scholarship and number of credits towards your degree. Especially the credits are valuable, as you can get the maximum amount of scholarship money during your stay on a near daily basis, anyway.
Not being able to woohoo looks like a glitch to me. My sims do that all the time. However, I don't strictly remember the vanilla rules anymore, as I've been using mods that cover the topic and bed share rules for a long time now. Even during my vanilla visits, those sims had high enough charisma and LTR's to overcome all obstacles, anyway.