As I understand transmission can be changed to any value, but what about all the other sliders, that situationally save time? For example, in Kenya setting the cars suspension all the way to soft gains a substantial amount of time. Am I allowed to change the steering sensitivity to any value I want, to effectively remove the only downside on a car like Ford Racing Puma?

I don't know where it is said otherwise. You can change the settings of the car. Many runners change the settings for certain routes. But remember, the RTA time that counts. So the time in which you change settings of the car also counts.

However in time trial mode this time loss doesn't exist, so for those who are not playing championship this question remains unanswered. Ideally the ruling on single stage category should be updated.

It is allowed to change car setup.

If this is Moderator approved then the question is settled.

@AnyupansGaming Car setups can be modified to anything you want for both IL levels and the championship however in the championship any time you spend in menus is counted in RTA