Leave MMS Max Message Size blank.

MMS UA Prof URL: http://www.apple.com/mms/uaprof.rdf

Then, back out to the main screen of the Settings app by tapping Network and then General, and then Settings at the top left; this ensures your newly-entered details are saved. Now, restart your iPhone by holding down the Sleep/Wake button until "Slide to Power Off" appears. Restart your iPhone by holding down that button again--waiting until the phone finishes shutting off completely first, of course.

When you restart your iPhone, the missing Camera icon should reappear in the Messages app. It did for me. But when I attempted to send or receive multimedia messages, it didn't work. T-Mobile eventually suggested I contact Apple for support, and to Apple's credit, the company did walk through several steps with me to try to correct the issue. Successful MMS'ing still eludes my setup, but if we figure it out, we'll update this story.