Okay, I think I've found the cause of the second rounding bug above: at the end of the bigcomp function there's a correction block that looks like

    ...
    else if (dd < 0) {
        if (!dsign)     /* does not happen for round-near */
          retlow1:
            dval(rv) -= ulp(rv);
    }
    else if (dd > 0) {
        if (dsign) {
          rethi1:
            dval(rv) += ulp(rv);
        }
    }
    else ...

The problem is that the += and -= corrections don't take into account the possibility that bc->scale is nonzero, and for the case where the scaled rv is subnormal, they'll typically have no effect.

I'll work on a fix...  tomorrow.