Soleimani was in Iraq, not Iran. So Trump did not act against the territory of Iran.
I don't know how many hours, days, or weeks this strike was in planning, but not telling Congress was leak prevention. As for Miss Nancy's whine, Shampeachments have consequences.
According to an NYT article I won't bother to link - their credibility is less than dubious in matters Trump - planning started a week ago, after that contractor was killed. Soleimani had been allowed to move around the ME with impunity for years, so I guess killing that contractor was the last of many "straws". Those years of impunity also seem to have motivated an arrogant carelessness in Soleimani.
The plan for the strike was that if Soleimani had bet met/escorted at/from the airport by Iraqi government officials, the strike was off. But if not, boom!