Hamilton's rank was dated to 19th September 1862, but it was signed into being on 10th March 1863, having been confirmed the day before. Hamilton is signing himself as BG up until ca. 20th January 1863, and signs himself as MG for the first time on 21st January. This would be as soon as the nomination is known.
In December 1862, Hamilton is still a BG(V). When Hamilton's seniority is settled in March, he is the ranking officer in 17th Corps. He made a claim for corps command, although the law as it stood then allowed POTUS to appoint anyone within the highest available grade to command, regardless of seniority (with regular rank being considered a different grade to volunteer rank). Grant reassigned him to 13th Corps, as McClernand ranked Hamilton, but Hamilton refused and resigned.
I don't see Halleck having any qualms about assigning Wallace to Grant's dept, but the VP really wanted Buell tried, and that needed two generals of equal or superior rank, and so Wallace and Ord were diverted to that duty.