Can the linea alba be shortened without surgery? (e.g. via exercise, and if not, what it the missing mechanism?)

A rectus diastasis is a separation of the abdominal muscles.  Exercise can strengthen the muscle but will not generally bring them back together (although you have proved this to be false).   With exercise perhaps there is increased muscle tone that compresses the abdominal contents resulting in less bulging reducing the diastasis.  Have you lost weight from exercise?  Perhaps this also could reduce the bulging and reduce the diastasis.  With a diastasis of 3.5 mm you do not need a tummy tuck, unless there is skin laxity.  Exercise will definitely not tighten skin.

Everyone's recovery after pregnancy is different.  It all about tissue quality.  Some women have great quality skin and fascia which has a better chance of returning to the pre-pregnancy form, while others don't have the elastic recoil and their skin becomes loose with stretch marks and have large diastasis recti.Think of the linea alba  and the lower abdominal skin like a older rubberband.  Each time you stretch it, there is less and less recoil.  Lucky for you, it sounds like your skin quality is pretty good and the distance between your muscles has decrease.  Unfortunately it is unlikely to totally return to your pre-pregnancy position even with exercise.Regarded the inevitability of a tummy tuck, in the end it is totally your choice.