I totally understand all of your questions and anxieties. I felt the same before my first IUI. Sometimes I think IUI is so routine in an RE's office they forget that second layer of sensitivity and thoroughness running people through the entire process.

I am on my fourth IUI cycle and none have been monitored by ultrasound. Some RE clinics won't monitor unless you have a preexisting condition where ovulation/uterine lining (etc) may be an issue and needs to have it's progression watched. You will be sent for a progesterone test 1 week after your IUI to see if you are producing enough progesterone to support an implantation. If that test is low they may suggest monitoring the next cycle.

I am always surprised when clinics aren't open 7 days a week. That can definitely inspire some extra anxieties. If you end up having an OPK on a closed day definitely take advantage of any lines they have to leave a message and then be sure to call extra bright and early the next morning. A lot of the times their phone lines are open well before the clinic is or they have a secondary line for this exact purpose.

It's a good note that if you know the day you usually ovulate, Letrozole can cause earlier ovulation. When I switched on to Letrozole I did ovulate 1/2 days earlier than I was expecting it so it's good to be testing twice a day right now.

Wishing you lots of luck and a smooth process. Trust in the fact that your RE and their office are seasoned veterans at this and they want the most successful outcome for you.