However...

However, according to the Imamiyyah school, ihram before miqat is not permissible except for one who intends to perform the `Umrah in the month of Rajab and is afraid of missing it if ihram is delayed until miqat is reached, and for one who makes a vow ( nadhr ) to assume ihram before the miqat.

( al‑Tadhkirah, Fiqh al‑Sunnah ) Ihram after Miqat There is consensus among all the legal schools that it is not permissible to cross the miqat without ihram, and one who does so must return to the miqat for assuming ihram. If he does not return, according to the four Sunni schools, his Hajj is correct though he should offer a hady in atonement. But if there be any impediment, such as fear of insecurity on the way or shortage of time, there is no sin.

This, regardless of whether there are other mawaqit before him on his path or not. According to the Imamiyyah, if he has deliberately neglected to assume ihram at the miqat while intending to perform the Hajj or the `Umrah, if he does not turn back to the miqat, there being no other miqat before him from which he can assume ihram, his ihram and Hajj are invalid, whether he had a valid pretext for not returning or not.

But if his failure to assume ihram at miqat was on account of forgetfulness or ignorance, if it is possible to return, he must do so; but if it is not possible, then from the next miqat before him. Otherwise he ought to assume ihram as far as possible outside the haram of Makkah, or within it; though the former is preferable.

( al‑Tadhkirah, al‑Fiqh `ala al‑madhahib al‑'arba`ah ) Ihram before the Hajj Months According to the Imamiyyah and Shafi'i schools, the ihram before the months of the Hajj is invalid if assumed with the purpose of Hajj, though it is valid when assumed for the purpose of the `Umrah.

Also, there is no difference of opinion that ihram is the first act of the pilgrim, irrespective of whether his purpose is `Umrah mufradah, or any of the three forms of Hajj. There are certain wajibat and mustahabbat related to the ihram.