She was in love with the former owner - she needs more than training - a reason to want to/accept it. She is willing to dick around with the MC thanks to Maria but for the most part it's entirely just for that - dicking around. To involve her in more is to have her accept wanting more from the MC that her former lover was giving her.
A couple things, first, if she is willing to suck his dick in the lobby, and have sex with him, she is willing to go further, it is just a matter of time. Speaking of time, how many years does it take? There was enough of a break between Grandpa dying and us arriving that they sold all of the other slaves, other than Lin, who was new at the time (quite a coincidence there), and now I have been at this hotel for nearly 3 years! We all need to move on eventually.
The other thing, is that she is clearly attracted to the MC, she has more than shown that, she felt the call of the Magic MC Dick, but it is always hard to transfer from one MC to another. But she is the first one you meet (unless you count that poor flight attendant from the new intro), and one of the last ones we fuck, so that is not an issue. If this were an anime, being the first one he meets in the game, would make her the OTP.
And to be fair, if she feels weird about being our sub, she would just as likely feel weird about being our dom. Don't get me wrong, she would look hot as a dominatrix, but personally, I am old, I don't like being pushed around, I don't like being blackmailed, and generally hate being told what to do by someone acting like they are better than me, if anyone is going to be domming someone in this relationship, it will be me, not the other way around. If she wants to dom someone, she can play with the other girls too, she can help Lin in her position as head of the harem.