Concerning your first question the Daas Zekainim MiBaalai HaTosfos (Parshas Terumah 25:5) proves that the Atzai Shittim were very light wood, since the 8 bulls given to the Bnai Merrari were able to carry the 48 Kerashim,
and their Adanim, the Amudim and Adanim of the courtyard ,the 9 Amudim of the Mishkan and their Adanim.
How this fits with the Gemorrah in Shabbos which seems to say that only the Kerashim and Adanim were on the wagons, has been bothering me for years.
I am not sure what the question is. Why can't it be Kadshai Bedek HaBayis according to Rashi and Ohr HaChaim ? Even if Moshe did not want to take them right away? The Ramban does use the words "Korban L'Bedek HaBayis"