I completely understand your standpoint and what you hope to achieve with minimal risk. ExpertFlyer is helpful in terms of viewing C space, but in my experience, I have been clearing the waitlist at same time as no C space opening up on EF. Consequently, I haven't received the saved alert notification from EF either. I just happen to check my AA booking on the app nightly and I notice my booking has changed with an AA ticket exchange email usually arriving within 24 hours. I have a theory that perhaps if the waitlist clearing is actually efficient that C space only shows up on EF when everyone on the waitlist has already cleared and theres still space.

With that said, I don't think the waitlist clearing process is perfect because I have been on the waitlist and received a C7 alert from EF and I've ended up calling the EXP desk to confirm space. The best strategy is to check saved alerts manually since emailed alerts aren't instantly sent. I check in the morning and then before bedtime weeks prior to the flight. In the week prior, I literally check after each meal out of mere habit. The last two flights Ive taken have cleared on the waitlist to my surprise with no EF alert whatsoever.

You most likely will not be able to secure C space at time of booking unless you are booking very close to departure (72-48 hours prior). Upgrades into First are a different story with space scattered throughout the month well ahead of time.