Cerberus got the plans for the Stealth Drive Simply by asking for them. REMEMBER: Until the Events of ME 1, Cerberus was a Black Ops Alliance sponsered Group that actually helped get the drive built in the first place.
Engineer Adams also states that, Given a month on board, Tali would know more about the engines than he did.
Salarians could have scanned it after Virmire. Commander Rentola might have (since it IS possible to kill Kirahe).
Quarians could also have simply been given the plans from the Turians in return for some assistance. Salarians also might have done so.
As for Tali, one of the engine Room Banters REALLY puts Egg on her face about it in ME 3.
And Shepard didn't steal the Normandy SR-2. EDI did. Or did the third Log about her on Chronos Station not ring a bell?