[Dawnguard Prophet Doesn't Start]

I recently had this same problem myself. I know how frustrating it can be, so I'm posting this so if anyone else has this problem they will know how I solved it.

 

To fix this I had to start a whole new game, and these are the only mods I had activated:

 

Skyrim.esm

Update.esm

Unofficial Skyrim Patch.esp

Dawnguard.esm

Unofficial Dawnguard Patch.esp

HearthFires.esm

Unofficial Hearthfire Patch.esp

Dragonborn.esm

Unofficial Dragonborn Patch.esp

RaceCompatibility.esm

SkyUI.esp

AchieveThat.esp

RaceMenu.esp

The Ningheim.esp

RaceMenuPlugin.esp

SkyRe_Main.esp

RaceCompatibilityUSKPOverride.esp

Civil War Overhaul.esp

I wish I could say exactly which mod or combination of mods causes this problem, but starting a new game with this relatively basic setup solved it for me.

[Why I believe the NCR is the best option]

I don't believe the knowledge of building new securitrons is lost. At least not to the Courier. At Big MT there are clearly facilities that can manufacture advanced robots (Dr. Mobius' robo-scorpions were built on site). There is even a reasonably well preserved facility specifically dedicated to deconstructing and analyzing securitrons. It does not take much imagination to think that those facilities could be salvaged and/or retooled to be able to manufacture new (or even upgraded/improved) securitrons.