ELEN=64b here. It can't be smaller because SEW=64b is listed. It can't
be larger because SLEN=64b
LMUL=1/8, SEW=32b is illegal
LMUL=1/4, SEW=64b is illegal
LMUL=1/8, SEW=64b is illegal
The compiler knows these are illegal because it knows ELEN.
Note that ELEN > SLEN has long been allowed in the spec (although it's expected that such implementations would be rare and unusual). But yes, if ELEN=64 then the natural constraint we independently proposed (LMUL >= SEW/ELEN in your words) would mean
those combinations of LMUL and SEW are illegal.
Conversely, if ELEN=128 (so that LMUL=1/4, SEW=32 would be legal legal), then we're looking at an implementation with ELEN > SLEN. Such implementations are somewhat odd and the usual intuition about SLEN's effects on the datapath doesn't apply to them
even at LMUL = 1.