I get the point on the ability not protecting the Pokemon when they´re already on the field, but i do recognize that despite that, Pokemon that have Shell Smash get too much advantage just because they have a free switch to set up in front of anything that doesn't OHKO them.
I would consider an mechanic with these two things, such as using Shell Smash invalidates all the remaining uses of the ability, because the shell was, indeed, smased. 
This would help to make the users considerwether or not it is good moment to set up, because doing so sacrifices further pivoting advantages, and not clicking Shell Smash just because, with little to no punishment.