I think it would go like this if I understand White Herb mechanics correctly:
-Opponent attacks you with a physical move (unless they're like Conklederp and slow as mud).
-White Herb instantly activates and negates the Defense drop. The Speed boost is normal.
-You use Shell Smash, and proceed to raise and lower your stats as normal, leaving you at +2/+2/+3, and -1/-1.
Would be fun, but White Herb activates as soon as a stat is dropped, not at the end of a turn.
-Opponent attacks you with a physical move (unless they're like Conklederp and slow as mud).
-White Herb instantly activates and negates the Defense drop. The Speed boost is normal.
-You use Shell Smash, and proceed to raise and lower your stats as normal, leaving you at +2/+2/+3, and -1/-1.
Would be fun, but White Herb activates as soon as a stat is dropped, not at the end of a turn.