Let's say person A is a Shi`a Muslim and has his permanent residence somewhere outside Makkah. He needs to visit Makkah for a day for a business meeting (not during Hajj season). He does not have such meetings regularly. He can not extend his stay in Makkah.
He doubts he'll be able to perform Umrah al-Mufrada with the short time he has on his hands and can, at best, only find time to perform a few Rakahs of prayer in the Masjid al Haram and perhaps perform a single Tawaf.
What does he do?
I've heard of 2 options.
1) He must perform Umrah al-Mufrada under any circumstance as it is not permissible for a non-regular visitor to enter Makkah without Ihram.
2) He may enter Makkah without Ihram and may not perform the Umrah, but he can not enter Masjid al Haram. People who say this seem pretty adamant about their position and this is what I hear from people who have (or know someone who has) faced such a situation in the past.
From Sayyid Sistani's website:
Similarly, if a person has completed Umrat-ut-Tamatu' and Hajj, or after Umrat-ul-Mufradah and has left Makkah, he is permitted to re-enter without ihram before the expiry of the same month in which the Umrah was performed. As for the person who has departed from Makkah after performing Umrat-ut-Tamatu', but before the pilgrimage, the rule will be discussed under Rule 154 below.
Assuming both the opinions are correct, the only way I see of reconciling the two is that Makkah = Masjid al Haram. That is, when the statement from the website says it's not permissible to enter Makkah without being in the state of Ihram, it doesn't mean the city Makkah but rather the Masjid al Haram.
Don't ask me to email my Marja.