I tend to agree with most of what you say, though I think M7 is not quite a "train wreck". Yes, the tempi are agonizingly slow, but the only benefit of that is at a slower tempo, the faster value notes of some of the inner orchestral detail is heard, which at faster tempi in other recordings, isn't as readily heard or taken in.
And yes, Schwarzkopf as the soprano soloist in the finale of M4 "isn't right". She's basically in the same camp as Lisa Della Casa in Reiner's M4. Both singers are Strauss singers, which isn't right for Mahler. The four best soprano soloists I've heard in M4 finale are Judith Blegen in Szell's Cleveland, Desi Halban in Walter's NYPO, Reri Grist in Bernstein's Sony NYPO, and Kathleen Battle in Maazel's VPO. This music requires a very light voice, which all four women fulfill wonderfully.
Wade