I've played tournaments on clay in over a dozen different countries in the last three years and I regret to inform you that your interpretation of the rules is incorrect.
it's been a long time since I needed assistance from a tournament official to rule out a situation on court, if I ever needed of course.
so, you might have fresh experience unlike me.
In the recent years, I faced mostly people that wouldn't escalate such things.
Moreover, everyone makes mistakes and line calling isn't different.
My experience was that taking the "soft" way is a much better way to solve a given situation and to have both players enjoy the match.
Typically something like: "Could you take a second look? ok, thanks, your side - your decision". This usually eliminates the tension and opponents tend to become generous with their calls.
It is not just a competition, but as well a social thing, and where I live and play it is an unwritten rule that after the match the winner buys a beer to both players and they chat over the beer.
So why spoil this with bad calling & cheating? you probably don't want people to avoid you.
And starting with a high enough ranking, probably something like the top 3% of players, the rules command an umpire. Which again leads to the question that was asked earlier, if at the beginning of the match there is no umpire, does it make sense to escalate such situation and call the officials?
Now, from my younger days.
Well, I shared my experience, which comes not only as a kid playing tennis, but much broader in tennis.
Glad for all the people here that experienced situations when there is no mark on a clay surface and their call stands.
Keep my fingers crossed that their experience doesn't turn-around and doesn't translate into some psychological traumas.
As someone mentioned above, and as I wrote, I have yet to see a situation when there is no mark on a clay court.
And I have yet to see a situation when the official comes and confirms that the call stands in a disputed situation and there is no mark.