Yes to the continued non-platonic (at least sometimes) valence of friendship - you definitely see people calling their spouse "my best friend" these days, for instance.

Maybe Sayers had an unrequited crush on some girl at Oxford? I feel that even a blinkered biographer ought to notice an entire love affair (although... maybe not, who knows with biographers), but an unrequited crush could easily leave no record, or no record outside an ambiguous poem, anyway.