However, there is a bug that allows the player to seemingly complete both options. For this error state to occur, you must tell Hardin about the "Chain that Binds" but NOT tell him about McNamara's orders to find scouts/paladins/filtration parts. The player can then turn in the filtration parts, tell McNamara you've turned in the parts, and finally tell Hardin about the orders. This is the only order of events that works, as the "Chain that Binds" dialogue options disappear from Hardin after turning in the filtration parts.

This route generates more immediate XP, but leaves the two NPCs in an error state. They will, for the rest of the game, both only tell the player to come back later. This prevents you from continuing on to the next part in the chain.

Also, you must reach a certain level of 'trust' with Arcade Gannon before he will open his past up to you, the following are ways to gain trust:

* Become accepted with the Followers of the Apocalypse.
* Take Arcade to the Crashed Vertibird, he will talk about how he hasn't seen one in a long time before quickly adding it must have been in a book.
* Take Arcade to the REPCONN Headquarters where he will talk about the history of REPCONN and it's research into plasma weapons after being bought by RobCo, with an intelligence of 6 you can inquire where he learned about this but he will dodge the question.
* Taking Arcade Gannon into the Silver Rush in Freeside and he will start a dialog about plasma weapons. You can ask him how he knows so much about plasma weapons but he will dodge the question.
* Going to Camp McCarran after completing the quest There Stands the Grass and talking to Doctor Hildern about OSI will gain trust.
* Going to HELIOS One and speaking to the Fantastic, then assuring Arcade Gannon you'll reroute the power supply to Freeside.
* The most beneficial option is to visit The Fort while Arcade is your companion, then remark that you "just want to see what Caesar's up to and get the hell out", this will gain enough trust to start the quest.

http://fallout.wikia.com/wiki/Still_in_the_Dark

If you did it this way perhaps you can revert to an earlier save and not talk with Hardin after turning in the filtration parts?

Otherwise...



http://fallout.wikia.com/wiki/For_Auld_Lang_Syne

It says nothing about needing anything from BoS for his quest line, at the end of which you learn about power armor and even get some for free.

Honestly, I understand your frustration with these kinds of bugs, but these bugs aren't keeping you from using Google or the fallout wiki to find a work around for these problems.