sander,
All of the questions you have so far, have been answered some many many times, it is hard to answer a million questions, and answer them again and again for every new member, aqua regia once mixed is active (it is the gases created by the reaction of the two acids that do most of the work to dissolve the gold (not just the acids alone), this is why you cannot make a batch of Aqua regia and store it, and expect it to react to dissolve gold later (at least effectively), once you use Aqua regia to dissolve gold it is no longer Aqua regia, gold chloride (with other metal chlorides) with possibly some free acid depending on working conditions, then this solution is de-NOxed to rid free nitric and oxidizer, then you add a chemical (or metal) to precipitated gold (depending on which chemical or metal you use will depend on the chemistry involved) but it is definitely no longer aqua regia, more of an acidic salt solution of chlorides and sulfates and...
My opinion using Urea can be dangerous (even in the waste solution later), and can give problems with certain metals, if you understand how to use aqua regia you will have no need for urea, and if you need to de-NOx the solution chemically, sulfamic acid would be a good choice, as it is safe and has benefits in the process, to help remove lead from solution.
As far as adding so much urea to effect the pH to where you could not precipitate gold, I do not think that could happen, actually urea will not destroy nitric acid, (it will destroy NOx gas), depending on how you used your acids it could or could not work at all, although a dangerous solution of urea nitrate can be made in solution.
Urea can also decompose to form ammonia or ammonium solutions, which can also become a danger with many metals, (even in the waste stream), and urea can complicate some recovery of precious metals like the platinum group.
The whole idea of using Aqua regia is to refine the gold (not to recover gold, for that you need to use a different process) so why would you want base metals involved with aqua regia? Base metals for the most part should have been removed from the valuable metals as much as possible before aqua regia is even used, and why would you want to add a chemical that would add more contaminates to the solution you are trying to remove the contaminates from, A chemical that will add these contaminates that may or may not work to get what you are trying to do done (like removing free nitric acid from the gold chloride solution), why wash the baby in muddy water?
Keep reading and searching for those answers, you will learn much more about these questions, and many more things you have, and answers to questions you have not even come up with a question for yet, most of the the answers will easily be found with some study.
If I have a question and ask it in a post I may get one partial answer, and some opinions, (Example: My question is fire hot, how hot, will it melt gold? First answer: Yes. Second answer: Sure it is hot, and it is hot enough to burn you. Third answer yes, you can melt gold with fire).
This answered my questions but I did not get much of an education by fire, or about fire.
But if I search for the answer to my questions, I learn many things to answer my questions, I can even learn much more than I would if someone just gave me a simple answer, and I can keep studying my question to learn even more about this subject, until I am satisfied, and ready to study my next question. I almost never ask questions with something I wish to learn, I find a source to study, and study to find the answers, so I can answer my question and I can learn more. If I limited my study to a few questions with partial answers that is all of the education I would get, but If I study myself and find the answers I can get a very useful education, that I would not have gotten from a few questions and half baked answers. Oh by the way fire is hot and it can burn you, and if you get it hot enough you can melt gold.
I hope this helps to answer a little bit of your questions, but more important it help you to get started on finding the answers to your questions through study where you will get more of an education on these subjects, than if you just got one half baked answer from me or others.
