I agree with the first suggestion. At the moment, securities are allowed to abandon the president if they want to get full armor, and they usually spend a number of minutes during which they are of no use at all for the president. They should be either disallowed to abandon the president for full armor or get it from the beginning. And I like the second option, as the /duty rule is already very hard to enforce and we don't want to make it even worse.
About Enforcers, I like the idea. I'd suggest to slowly regenerate armor to the driver and all the passengers of the Enforcer. Otherwise, players would bring them to fights and quickly enter them every time they are about to die, which would make them nearly invulnerable. Also, I would place them in SWAT spawns. As SWAT and cops spawn practically in the same place of every map (in except for NSA), I would move SWAT spawns further from police stations so it's not that easy for cops to take advantage of the Enforcers. After all, this feature is expected to encourage use of the SWAT class, not to empower cops and terrorists even more.