Apologies - didn't mean to make it confusing.
I honestly don't know if prespend is actually necessary but in my experience, I asked about the CTG before I made any purchases at that location. I visited the Sevres location twice:
1st time: I went to look at the space (it's really beautiful) and to mainly check out the cafe and bookstore they had there. I did head downstairs to where the appointment desk was and asked about the CTG and was advised to try tomorrow and earlier in the day. They confirmed that I didn't need an appointment. (My impression is that if they were to get any deliveries for items, it'd be earlier in the day rather than later.)
2nd time: At the desk they assigned me an SA and I indicated to him that I wanted a CTG, specifically. Fortunately, they had one left (or so they told me) and it was in one of my color preferences. I did end up purchasing some other items afterwards - but I did not get any sense of pressure from the SA that I needed to.
So at least for the CTG - I really don't think there's a need to purchase ahead of time/connect with an SA; if they have it, they will show it to you. I would just ask directly at the desk.
At George V they actually showed me 3-4 options (including one Kelly To Go), and I just walked up to the SLG section (no appointment needed).
There is no waiting list for the SLGs.
I hope that helps! And I'm so sorry to hear about your cancer treatment; I know that it can be extremely difficult and exhausting. Sending you positive thoughts and prayers, and much luck while in Paris.