When The Catalyst presents Shepard the three optional endings, the only one he alerts that Shepard will die is the "control ending".
In "destroy" ending, we see Shepard breathing. So why can't we suppose that Sheppard is alive the "synthesis" end as well?
He/she died alredy in the end of ME1, so anything is possible
The process of synthesis involved basically, breaking Shepard down to the DNA level & scattering him/her across the galaxy or whatever. Shep's mind wasn't downloaded somewhere along the way, so I don't think there's a way back from that.
When Shep died before; they had a body to rejuvenate. If you play the suicide ending in 2; Shep dies there as well and can't be revived because the station is either incinerated, or super-irradiated, destroying Shepard's DNA again.
Even the clone-Shep wasn't like the real one & had to have a bunch of neural implants just to know how to act human.
I remember the days when Kardashians were just the bad guys on Star Trek!