Yes, always consider the bystandar. TPO.
TPO? Is that actual English or something we made up here?
It stands for Time, Place, Occasion. Mind the context.
If it's bespoke they should have the patterns saved for you. Which means no problem in making another pair of trousers that fit the same. I would have no qualms at all about contacting the maker and asking that they make things right. What is the point of bespoke if you don't get what you want?
The most important in requesting a re-make isn't whether they're exactly the way you asked, but rather if you really care if they have belt loops or side tabs. If it's important to you, then they should be able to remove belt loops and make side tabs out of scraps of fabrics left from the cutting process.
If you don't really care that much, just make sure they know there was a mistake, and enjoy your pants.
I don't see the point of making them pay for a mistake if it isn't of any importance to you. Picking one's battles is as important as fighting them.