El Daddy has an excellent sticky thread [INFO] Nexus 5 OTA Help-Desk
which explains how to sideload an OTA to your stock/unrooted device. This is fully supported method provided by Google themselves. The OTA can be downloaded direct from Google servers and if it was tampered with, it wouldn't pass signature checks.
They have also had engineers explain how their OTA rollouts work, including expected time frames.
What all this means is there's no reason for us to try and reverse engineer this process anymore and therefore this thread's only purpose is chatter that amounts to nothing
moto x: 4.4 | flo: 4.4
other: Chromecast | Chromebook | System76 | MacBook Air
former: hammerhead | m7 | mako | Kindle Fire | CR-48 | Revue | grouper | maguro | crespo | passion | GT-I9100 | vision | rhodium | kaiser