1. Lies to the MC (about dinner with Sebastian being for work).
She does lie to MC about dinner with Sebastian, and I think anybody will concede that this is potentially problematic. The question is "Why does she lie about it?" Her "spidey sense" is tingling, and she's not quite sure what Sebastian is playing at yet. However, she DOES want the business relationship to be successful so she decides to go to the restaurant even though she is uncomfortable about it. Does that make her a whore? I don't think so. I think it makes her a driven artist who wants to achieve success, and she has the opportunity in front of her. She's aware of a potential pitfall but decides to take the risk with eyes open. I don't think it's so surprising that she's hesitant to share all of her thoughts on the matter with her husband at this point. Nothing has happened and her unease may prove to be unfounded.
It's not unlike MC's decision whether to tell Louise about working at a strip club. One slight difference though is that the Purple Orchid IS a strip club. There's no question, and Louise WILL find out about it eventually. In Louise's case, whether anything happens or not is dependent on whether MC is supportive or not. That is, if MC is supportive, Louise feels comfortable nipping inappropriate Sebastian's behavior in the bud since if that results in the deal falling through, she is comfortable that MC will support her as she looks for another opportunity.
TL;dr She lies to MC but it is entirely reasonable to assume that she does so to keep MC from worrying while she figures out what Sebastian is up to. That assumption is just as backed up by what we know from the game as yours. Bottom line is that we don't know exactly what she was thinking because we are not told directly.
2. Dresses sexy for the dinner with Sebastian (and no this wasn't just an adult VN thing, she actually thinks about it.)
This is how she dressed. I'd say it's flirty rather than overly sexy. It doesn't actually show much skin. My objection to this outfit would actually be more that it is younger than her actual age if you catch my meaning. (The game suggests this with a narrative comment:"(Sebastian) turns around. Leaning against the doorway stands [l]. To look at her you would think she was twenty-one again.") However, she's an artist, and it's not unusual for an artist to have different fashion sensibilities.
We are not shown any scenes of her getting ready, so we do not know what she was thinking while getting ready. The line you seem to be alluding to is: "(Did I go too far? {w} This is not a date...)" which she thinks right after Sebastian compliments her. Rather than her planning to dress slutty, this actually suggests to me that she didn't think too much about what she put on until right that moment when it was too late to change.
3. Wonders to herself just how far she's willing to go with Sebastian for this job.
As I said before in response to somebody complaining about the MC thinking about how hot his nurse was, thoughts are not cheating. Thoughts are thoughts. She wants this business relationship to succeed. She's a bit unsure about what Sebastian wants, but if he might be interested in her, it isn't remotely surprising that she question herself in this way. Asking the question does not mean she intends to take advantage of the situation. It just means she acknowledges the situation.
By the way, I find the line in the script where she asks herself this question to be interesting. It says: "(Just what am I willing to do for this... {w}for them.)" Even at this moment, she wants to make her family proud of her. If she is not supported and decides to stay for the rest of the dinner, she says: "I know what you are saying, . I will do what it takes." And she thinks immediately after: (For them...). Messed up? Maybe, but so is the unsupportive home environment that lead to the situation. It also still doesn't mean she intends to fuck him.
Ultimately, the only thing she can be judged on is what she actually does which depends on how MC has treated her so far. As I said above, if MC is supportive, she is more secure in putting Sebastian in his place if he steps out of line. If Sebastian kisses her, it is clearly non-consensual. We don't get to see how Louise handles it with Sebastian so we can't judge her on that. She doesn't tell MC about what happened until well after he is released from the hospital after the accident. That could be considered problematic, but interestingly, in the case that Louise has been kissed by Sebastian and MC turned Cindy down at the Purple Orchid, MC doesn't tell Louise about what happened either.
EDIT: Goddamit, why has everything gone strikethrough? Grr...
EDIT2: Okay, think I fixed it... Apparently the character name dereferencing syntax from the script is the same as the strikethrough meta-character. Oof!
