The answer to these two questions lies in the respective exegetical approaches of these two commentators. Ramban's approach throughout Chumash is "yeish mukdam u'muchar b'torah," meaning that he operates with the basic assumption that the sections of the Torah are written in chronological order. The Ramban makes exceptions to this rule only on the rarest of occasions where there is no other possible explanation. In his opinion, the similarities between the Mishkan and Chet Ha'egel are not strong enough to 'override' his basic assumption that the parshiot, or sections, need to be in order.
Rashi, on the other hand, maintains "ein mukdam u'muchar b'Torah." His basic assumption is that the location of parshiot of Mitzvot throughout chumash does not necessarily reflect when the Mitzvot were given; rather, they are written in thematic relation to the ongoing story. Thus, according to Rashi, even though the parshiot of the Mishkan in Terumah were actually given to Moses after Chet Ha'egel during the second set of forty days, the Torah places them immediately after Har Sinai to emphasize that the Mikdash is a perpetuation of Sinai.
Therefore, Rashi and the Ramban are in total agreement as to the fundamental importance of the Mikdash as well as to the thematic connection between Har Sinai and the Mishkan. Their disagreement arises from a dispute in their exegetical approach, i.e. the degree to which Chet Ha'egel affected the construction of the temporary Mikdash - the Mishkan.