I think brother vega answer most of your questions.

But you know it depends into what sect of Muslims you ask to, as for myself, i had mentioned that i am sunni, so i would definitely say yes! its illegal, speaking of sunni Islam.

Actually the proper words to say is haram or halal, And in sunni sector, we consider it forbidden and we dont even promote that kind, to which contradict to shia's belief. But, whatever reasons they may have...we are obliged to respect.

Let me site you a situation of temporary marriage:
(Dear all, no offense to this post as i'm just trying to explain temporary marriage, everyone is welcome to support their opinions here)

*A guy named Mr George, came to Qatar on a tourist visa, and somewhere then met another man named Mr. John who apparently has unmarried daughter, and assuming she is a virgin
(not necessary).
Then, Mr. George propose to Mr. John to temporary marry his daughter during his stay to be specified in the marriage contract 2 months, he will give her dowry, provide her house, clothes, foods and all her rights she possess. However, its for a period of 2 months only.

My question, if you are Mr. John..will you allow your daughter to engage in that temporary marriage?

Dear silver, plese consider my question as friendly.

Kind regards,
Apple