I'm not seeing any conflict of interest - you were not interviewed by another applicant, since at the time of your interview the senior manager was not an applicant. When they became an applicant, they were removed from the process.

It might have looked better if people who had been interviewed earlier by someone who later became an applicant were re-called for another interview - if this is a policy you're suggesting HR adopt in future, that might be a useful suggestion.

If you believe you would be better at the new job than the senior manager, you could deal with it in exactly the same way as you would if someone else had been appointed. That said, this approach is unlikely to get you anywhere, and will look like you're trying to make trouble for your new line manager.

But it's not wise to mix up the two - if you're hoping HR will adopt a more thorough policy in future, you don't want it to appear that you're questioning the CEO's choice on this appointment - HR will assume you're acting out of self-interest and not for the benefit of the company.

I'm not seeing any conflict of interest - you were not interviewed by another applicant, since at the time of your interview the senior manager was not an applicant. When they became an applicant, they were removed from the process.

(It's possible that the senior manager will have been privy to any "ideal" answers at the start of the process, will have heard the answers given by any candidates before they decided to apply, and may have expressed an opinion to the CEO on earlier interviews, but the CEO will also be aware of what they knew - so they're unlikely to have got away with using this information.)

It might have looked better if people who had been interviewed earlier by someone who later became an applicant were re-called for another interview or that the interview process was restarted - if this is a policy you're suggesting HR adopt in future, that might be a useful suggestion.

If you believe you would be better at the new job than the senior manager, you could deal with it in exactly the same way as you would if someone else had been appointed. That said, this approach is unlikely to get you anywhere, and will look like you're trying to make trouble for your new line manager.

But it's not wise to mix up the two - if you're hoping HR will adopt a more thorough policy in future, you don't want it to appear that you're questioning the CEO's choice on this appointment - HR will assume you're acting out of self-interest and not for the benefit of the company.