The Offertorio (Domine Jesu and Hostias) is fantastic - I don't think it's boring at all. Mozart wrote out the vocal parts for virtually the whole requiem - any decent musicologist on the basis of that alone - could construct the instrumentation around it because the first violin part is also written out.
As i said - the Sanctus is highly suspect - is second rate. Compare it with the majestic sanctus in the c minor mass - no comparison at all. The Benedictus - is beautiful - I think Mozart must have sketched that to some extent and S completed it. The agnus dei also is wonderful in its way and I doubt if S could have conceived that all by himself. S was a mediocre talent - there is no way he could have invented - the sanctus may be his alone - he did a decent job there. What modern composer would even attempt to finish off a master's work? It took D Cooke 10 years to complete Mahler's 10th. S had just weeks.
The best parts of this work are so fabulous it is worth listening to and coming back to.